Weird things with .22LR match ammo

[grauhanen]

grauhanen, Were your shots at 100 done with 'cooling time' between shots ? I noted in the first set of groups a couple with 'same mv' hitting at opposing heights, with 10 or 20 rounds (depending on your target sequence) between them. Short time gaps causing a warmer barrel effect due to diff viscosity or moistness of lube or thermal expansion of the barrel ? jaia has at one time suggested that 'moisture effect' as a possible variable . I don't recall if he had substantiated that or just posited it.

Unlike centerfire and calibers such as .17HMR, "cooling time" for .22LR isn't usually a thing for target rifles. BR and ISSF (Olympic-style) shooters don't worry about it. In this particular case, a single shot bolt action .22LR shot for accuracy is difficult to shoot rapidly. It takes about half an hour to shoot 50 rounds. This pace can't cause significant barrel warming. Furthermore, the cool fall morning temperatures can't contribute much, if anything, to warming.

For anyone interested in learning more about .22LR barrel temperatures, there was a study on barrel temperatures on .22LR target rifles which showed that with a rate of two shots per minute a barrel warms near the receiver only by 5 degrees Celsius, and much less further down the barrel. As far as a warming barrel changing bore dimensions, the study calculates that any change due to temperature would be "insignificant." According to the study, such warming of the barrel as may occur doesn't affect the POI. For more details, see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3944560/

There is something to be said for maintaining consistent fouling in the bore. Without sufficient initial fouling shots, the coating in the bore of both the lube and detritus of the fired round will not be even. This may affect accuracy performance. Fouling is normally wet and it dries after a period of inactivity. Shooters wishing to maintain accuracy will wish to maintain fouling consistency (and wetness). This may require shooting into the berm every so often -- or perhaps cleaning the bore and refouling as necessary.

 

[Raton57]

The difference in velocity is so small I am not sure what your point is .

 

[grauhanen]

The difference in velocity is so small I am not sure what your point is .

This is a good question for shooters unfamiliar with .22LR match ammo performance.

At 100 yards a small difference in MV with high MV centerfire rounds doesn't matter much. For example, at 100 yards two .223 Remington rounds that are 10 fps apart in MV are predicted by ballistics calculator to have a vertical difference less than 0.02". That's not much.

At 100 yards with the much slower .22LR bullet, however, a small difference in velocity can make for a more significant difference on target. According to ballistics calculators, every 10 fps difference in MV between rounds results in vertical difference of about 0.25". That's much more significant.

As a result, even small differences in velocity between .22LR match ammo rounds can have significant impact on POI.

Of course, the issue here concerns the "weird things" that are observed with .22LR, particularly with MV/POI mismatch. To put it another way, too often these bullets don't go where they should -- at least according to MV predictions. The question or point is why do they do that?

 

[Lead hammer]

Positive compensation.

 

[_Shorty]

The answer lies in the things we've already discussed in the tuner threads and the thread about my two-flats barrel. What the barrel is doing during shots, its harmonics, remains basically the same for all shots, even with different ammo. But no two shots leave at the same time. And when shots leave at different times the barrel will be pointed in different directions, and naturally this results in different launch angles. This can be mitigated to some degree with an appropriate tuner. This is a huge factor.

Another factor is the BC of each bullet. Every shot has a different BC that can vary quite a bit, given we are talking about soft lead bullets. They're not made perfectly. And they can be altered a fair bit between the time it leaves the factory and makes it into your chamber. If you've shot a fair bit of Eley you've no doubt had to dig the bullet tray out because it fell inside the box, and the bullets were then no longer suspended in the air hanging from the tray, but were bashing around against the bottom/side of the box the whole time they were being transported. I can share quite a bit of BC data if you'd like. I have a bunch of Eley Team data, and some Tenex, and some Club, and some Club Biathlon, and once I can re-identify which runs were Lapua Center-X and Midas+ I have some of that, too. And CCI Standard Velocity, Clean 22, and Pistol Match. And RWS Club Master, and Semi Auto. But even just looking at one you can see how every shot varies, and can vary quite a bit sometimes. I use a util called Labrabaco to calculate the BC of each shot, and it defaults to excluding any shots from the average result if those shots vary by more than 2x SD, and a bunch of different ammo seems to have around 5% or more excluded because of that. It also lets you choose a slightly more aggressive filtering where it excludes anything that differs by larger than 1.64x SD, and this seems to bring the excluded bunch to around 10% for a lot of ammo. I've also seen some rather poor shooting ammo pass that filter without exluding any rounds at all, so the poor results on paper are due to some other factors besides BC variance since they were all fairly similar.

I think what it comes down to more than anything is simply a difference in where they were aimed. And that's barrel harmonics versus exit time. Rounds that vary a lot in how they burn will vary more on paper. Rounds that have a smaller amount of variance in how they burn will vary less on paper. And what's likely the largest reason why is because when exit times vary less, where they are aimed will probably vary less. And you can influence the amount of aiming variance via a tuner, too.

 

[Chilly807]

With regard to barrel temp affecting impact, the problem with a .22 lies in keeping it warm, because it cools down at the drop of a hat. Just walking 50 yds to change targets is enough to make a difference. I usually fire a couple into the backstop before I go "on score" again. I used to fire them into the sighting bulls, but they're really not even sighting shots, they're conditioning shots to get things back to a steady state.
Why treat them as sighters when they're not reliable?

I also shoot centerfire F-Class, where you usually get 2 sighters and 10 on score per relay. I used to use rounds that were less than perfect in some respect as foulers/sighters.
Then it occurred to me that of all the shots that need to be perfect, the sighters go right to the head of the line. You're adjusting your scope, or hold-off, to reflect "average" conditions that you can repeat for the rest of the match. So, using ammo that's less than perfect for those 2 critical shots sets the stage for problems when you do go on score.
Now when I shoot a rimfire match I try to start with a fouled barrel, and fire the first 5 off target to get things warmed up slightly. Then come sighting shots, and when I'm happy with that I go on score. Most of our .22 matches are 30 minutes for unlimited sighters and 20 shots for score. It seems like lots of time until you get a gusty day when you're waiting for the wind to settle, or you have trouble getting a zero on the first target.

 

[grauhanen]

The answer lies in the things we've already discussed in the tuner threads and the thread about my two-flats barrel. What the barrel is doing during shots, its harmonics, remains basically the same for all shots, even with different ammo. But no two shots leave at the same time. And when shots leave at different times the barrel will be pointed in different directions, and naturally this results in different launch angles.

If two rounds with the same MV can't leave at the same time and would have different launch angles, this means that MV is not relevant to exit time and launch angle. In that case a tuner would be rendered utterly irrelevant.

Two rounds with the same MV ought to leave at the same time, with the same launch angles. As a result they should have very similar trajectories and POI, providing all else is equal (including, for example, bullet symmetry and shape, no wind).

Yet this is often enough what's not seen. Rounds with the same MV don't always have similar POI. Furthermore, sometimes slower rounds have POI above faster rounds and vise versa.

 

[yodave]

pull 10 loaded rounds apart and weigh the bullets and primed cases, just because you have the same velocity on 2 shots means very little when the first bullet weighs half a grain more then the second bullet, you're taking way to far for granted that the 40 grain bullet is in fact 40 grains. The more you pay for ammo, the closer in weight the bullets are, but even Eley Match has some very light and very heavy bullets that stray far from the 40 grain label. And, by all rights, a faster bullet is going to hit lower on the target as the exit time on the barrel is that much sooner then the effects of the barrel rise due to recoil, if your shots with a black powder rifle are to high on the target, you add powder to speed up the shot to get it out of the barrel before you steer it upwards.

 

[_Shorty]

If two rounds with the same MV can't leave at the same time and would have different launch angles, this means that MV is not relevant to exit time and launch angle. In that case a tuner would be rendered utterly irrelevant.

Two rounds with the same MV ought to leave at the same time, with the same launch angles. As a result they should have very similar trajectories and POI, providing all else is equal (including, for example, bullet symmetry and shape, no wind).

Yet this is often enough what's not seen. Rounds with the same MV don't always have similar POI. Furthermore, sometimes slower rounds have POI above faster rounds and vise versa.

This would only be true if the primer and powder burns were always exactly the same rates. You can have faster-primer/slower-powder for shot 1 and slower-primer/faster-powder for shot 2 and still have both exit the muzzle at 1085 fps. But this doesn't mean that the internal ballistics for both shots was the same. They can have different acceleration curves that still result in the same exit velocity. And that would result in different exit times with the same exit velocity. As Dave mentioned, some other variables also exist that can contribute to the same exit velocity but different exit times, like bullet weight. I just weighed a brick of Eley Club the other night, and the round weights ranged from 50.5 grains up to 52.5 grains. In the past I also pulled bullets apart, as he suggested, and weighed the components. I believe I pulled apart a whole box of 50, probably Eley Target at the time, as it was quite a few years ago. The bullets were responsible for the vast majority of the weight difference then, and I'm sure that holds true now. The cases didn't vary all that much, but did vary some, and the powder charge was so small it was hard to really differentiate between them when using a 0.1-grain scale. The bullets were responsible for the vast majority of the weight difference from round to round. That's going to affect how much acceleration there is, on top of the fact that there are likely going to be very small differences in the amount of primer compound and powder present in the case, as well as variance in their burn rates due both to the variance in the amounts present and the fact that they're mixtures that won't always burn at the same rate from one example to the next. You might still get 1085 fps from a 39.5-grain bullet and a 40.5-grain bullet, but chances of them exiting at the same time are small. There's likely to be some difference in exit time, and that difference in the amount of time that has passed is going to result in a difference in POA at the muzzle. And when you amplify that out to 50 yards or 100 yards it doesn't take much to make a noticeable difference on the paper.

And none of that negates the possibility of a tuner working. But this brings us around to something that always needs to be mentioned. Tuners aren't magic. You don't throw one on and break world records with CCI Standard Velocity. It is still going to vary like CCI Standard Velocity does. You will be able to improve the results you see somewhat, but you're still bound to the nature of CCI Standard Velocity. There's way more variance in that stuff than there is in Eley Tenex. And the variance from round to round being smaller in Tenex in all the various components is what helps it shoot better than CCI Standard Velocity. Tighter manufacturing tolerances leads to less variance in shot behaviour. Less variance in shot behaviour leads to more consistent results on paper. And the more consistent it is to begin with, the better it will get along with the more controlled behaviour that a tuner can introduce. I have absolutely no doubt that playing with a tuner that's suitable for a given gun will improve the results seen with any ammo. But you're still talking about improving upon a certain performane baseline by a reasonable amount. You're not talking about taking any performance baseline and making it perfect just by controlling the barrel behaviour to some degree. Really good results are still going to require really good ammo. I'd say it is a good bet that two 1085 fps rounds of Eley Tenex are more likely to exit the muzzle at nearly the same exit time than two 1085 fps rounds of CCI Standard Velocity, going by the results on paper.

edit: If you're not already familiar with it, look up Spearman's Rank Correlation Coefficient. This is something that can be applied to velocity versus vertical rather easily. It can tell you whether or not there seems to be a correlation between velocity and vertical, and how much you can trust that answer, when examining a target. It is another way you can tell whether or not what you're doing with a given tuner is helping or hurting, and whether or not you're moving in the right direction. And the funny thing is, as you get closer to the ideal tune, it starts helping you less and less, because it starts looking less and less like there is any relationship between the two at all. Luckily, though, the target score should begin looking good by the time the SRCC results start looking bad.

 

[grauhanen]

pull 10 loaded rounds apart and weigh the bullets and primed cases, just because you have the same velocity on 2 shots means very little when the first bullet weighs half a grain more then the second bullet, you're taking way to far for granted that the 40 grain bullet is in fact 40 grains. The more you pay for ammo, the closer in weight the bullets are, but even Eley Match has some very light and very heavy bullets that stray far from the 40 grain label. And, by all rights, a faster bullet is going to hit lower on the target as the exit time on the barrel is that much sooner then the effects of the barrel rise due to recoil, if your shots with a black powder rifle are to high on the target, you add powder to speed up the shot to get it out of the barrel before you steer it upwards.

Among the various .22LR cartridge components, weight differences between rounds is most likely associated with bullet mass being different. Bullet weight makes up about 78% of a .22LR cartridge's mass. And higher grades of match ammo should be expected to have bullets close in weight.

For the general reader, according to in George Frost, Ammunition Making, weight variation for .22LR bullets should be kept within .2 grains for non-match bullets, within .1 grains for match. (p. 29.) In his chapter on .22LR match ammo, Frost says weight control in bullet manufacturing should be within a range of plus/minus .05 gr. on any sample of ten bullets. (see p. 135.)

It's common sense that with all else between cartridges being equal -- that is the amount of primer/propellant, crimping -- heavier bullets will have a slower MV. Rounds with a slower MV can be expected to have a lower POI than faster ones. The exact weight of the bullet shot always remains unknown to the shooter.

 

[grauhanen]

This would only be true if the primer and powder burns were always exactly the same rates. You can have faster-primer/slower-powder for shot 1 and slower-primer/faster-powder for shot 2 and still have both exit the muzzle at 1085 fps. But this doesn't mean that the internal ballistics for both shots was the same. They can have different acceleration curves that still result in the same exit velocity. And that would result in different exit times with the same exit velocity. As Dave mentioned, some other variables also exist that can contribute to the same exit velocity but different exit times, like bullet weight. I just weighed a brick of Eley Club the other night, and the round weights ranged from 50.5 grains up to 52.5 grains. In the past I also pulled bullets apart, as he suggested, and weighed the components. I believe I pulled apart a whole box of 50, probably Eley Target at the time, as it was quite a few years ago. The bullets were responsible for the vast majority of the weight difference then, and I'm sure that holds true now. The cases didn't vary all that much, but did vary some, and the powder charge was so small it was hard to really differentiate between them when using a 0.1-grain scale. The bullets were responsible for the vast majority of the weight difference from round to round. That's going to affect how much acceleration there is, on top of the fact that there are likely going to be very small differences in the amount of primer compound and powder present in the case, as well as variance in their burn rates due both to the variance in the amounts present and the fact that they're mixtures that won't always burn at the same rate from one example to the next. You might still get 1085 fps from a 39.5-grain bullet and a 40.5-grain bullet, but chances of them exiting at the same time are small. There's likely to be some difference in exit time, and that difference in the amount of time that has passed is going to result in a difference in POA at the muzzle. And when you amplify that out to 50 yards or 100 yards it doesn't take much to make a noticeable difference on the paper.

It's possible that identical bullets with the same MV can be the product of faster primer/slower powder or slower primer/faster powder. The problem is that no one can know if -- or when -- this may be the case. There's no way for a shooter to identify and confirm or even measure different "acceleration curves" . He can only know the MV of the bullet.

Regardless of how these bullets arrive at the muzzle, they still have the same MV. Within the length of a barrel, say 24" - 26" or so, there's not a lot of room for variation concerning how two bullets arrive at the muzzle with the same velocity. In other words, since bullets don't accelerate after leaving the barrel, bullets with the same MV should have similar acceleration curves in order to get from a standstill in the chamber/leade out to the muzzle.

To return to the previous point, barrel vibration patterns are either always random and independent of MV -- or they are always related to MV. They can't be both at the same time. If it's the former, accuracy performance and consistency is all the more challenging.

The question remains, why some rounds have MV/POI match and some have MV/POI mismatch.

 

[Lead hammer]

Just because the mv’s can be the same, it does not represent how it got there. I am sure barrel vibration would be different, and has the potential to change poi. In centerfire, two different powders with the same velocity and the same components are not likely to have the same poi. I don’t think rimfire can be made perfect enough to have common mv as the only contributing factor to poi shift.

 

[Biologist]

. The cases didn't vary all that much, but did vary some, and the powder charge was so small it was hard to really differentiate between them when using a 0.1-grain scale.

I am just being a science nerd here, but that statement above is enough to render the investigation invalid (Re pulling bullets and measuring components, etc.). The primer and powder are THE energy source which accounts for all of the kinetic energy and resultant MV. If the assumption is that the weighing apparatus was not fine enough to measure true powder differences to the required significant number of grain decimal points, then we have a problem.

If one was testing for bullet weight variance, then the powder charge variable must be controlled for statistically. Sounds like a laboratory quality weigh scale is needed for the powder, like a Sartorius that measures to the 0.001 grams (0.0154 grains), or maybe to 0.0001 grams (0.0015 grains), accurately. How many decimal points are necessary? I don't know, but that can be experimentally determined.

Quantifying the difference in chemical energy load between every cartridge, to me sounds like Job 1 to develop the methodology for.

 

[grauhanen]

Just because the mv’s can be the same, it does not represent how it got there. I am sure barrel vibration would be different, and has the potential to change poi. In centerfire, two different powders with the same velocity and the same components are not likely to have the same poi. I don’t think rimfire can be made perfect enough to have common mv as the only contributing factor to poi shift.

The last sentence is probably the closest anyone has come to offering an explanation that begins to make sense.

Regarding the first sentence, there may be less sense. Of course shooters can't measure bullet velocity as it travels through the bore. But that's not necessary. With two .22LR rounds at the same MV, there's only one way can they go from 0 fps at the chamber/leade to their identical muzzle velocity some 24 - 26 or so inches away. It's not possible for the rate of acceleration in the same bore to be different for two rounds with the same MV.

Once out of the muzzle and not counting wind, a .22LR bullet's trajectories will be mostly determined by its velocity, mass, and symmetry. The forces of gravity are identical. Drag forces with almost identical bullets will be virtually the same too.

When muzzle velocity and bullet mass are virtually identical, POI should be virtually identical. But often they are not. And that's the crux of the issue.

Shooters can measure velocity and shooters can measure mass (or at least calculate reasonable averages after destroying enough rounds to weigh the bullets). They can also assess POI. There's only one major thing that affects trajectory that remains ungauged -- symmetry or center of gravity.

 

[_Shorty]

Among the various .22LR cartridge components, weight differences between rounds is most likely associated with bullet mass being different. Bullet weight makes up about 78% of a .22LR cartridge's mass. And higher grades of match ammo should be expected to have bullets close in weight.

For the general reader, according to in George Frost, Ammunition Making, weight variation for .22LR bullets should be kept within .2 grains for non-match bullets, within .1 grains for match. (p. 29.) In his chapter on .22LR match ammo, Frost says weight control in bullet manufacturing should be within a range of plus/minus .05 gr. on any sample of ten bullets. (see p. 135.)

It's common sense that with all else between cartridges being equal -- that is the amount of primer/propellant, crimping -- heavier bullets will have a slower MV. Rounds with a slower MV can be expected to have a lower POI than faster ones. The exact weight of the bullet shot always remains unknown to the shooter.

I don't know what ammo he was weighing, but nothing I've weighed had bullets that varied so little. The brick of Eley Club that I measured the other day looks like this:

I've measured a lot (many 5000-round cases) of ammo, and I've never seen anything with bullet weight variance as low as that guy's talking about, so I am afraid I call BS on that one. I have three muffin tins that I use for sorting, and they're half a dozen bins each, and are labelled in 0.1-grain increments. It is unusual if I have more than a handful that don't fit in those 18 bins, as was the case with this lot of Eley Club. It needed 20 bins, but barely. Most of the time those 18 bins are enough, and a small number of them are likely to go unused. Never have I ever measured a brick or case of ammo and only used 2-4 bins, which would be the case if I saw similar results to that dude. Usually ~15 bins is enough, and it doesn't go over 18 often.

It's possible that identical bullets with the same MV can be the product of faster primer/slower powder or slower primer/faster powder. The problem is that no one can know if -- or when -- this may be the case. There's no way for a shooter to identify and confirm or even measure different "acceleration curves" . He can only know the MV of the bullet.

Regardless of how these bullets arrive at the muzzle, they still have the same MV. Within the length of a barrel, say 24" - 26" or so, there's not a lot of room for variation concerning how two bullets arrive at the muzzle with the same velocity. In other words, since bullets don't accelerate after leaving the barrel, bullets with the same MV should have similar acceleration curves in order to get from a standstill in the chamber/leade out to the muzzle.

To return to the previous point, barrel vibration patterns are either always random and independent of MV -- or they are always related to MV. They can't be both at the same time. If it's the former, accuracy performance and consistency is all the more challenging.

The question remains, why some rounds have MV/POI match and some have MV/POI mismatch.

You don't need to know whether or not their acceleration curves were different. You only need to understand that it is possible to have different acceleration curves and still arrive at the same muzzle velocity. But you do not seem to be able to fathom that this is possible for some reason. Just like it is possible for two different cars to both cross the finish line in a 1/4-mile drag race at 150 mph but have very different elapsed times it is also possible for two bullets to have the same muzzle velocity but take different amounts of time to exit the muzzle. You still don't seem to be understanding barrel harmonics properly with this statement. Barrel vibration patterns aren't random. They're actually fairly consistent. What's not very consistent is when bullets exit the muzzle in relation to their vibration pattern. When your bullets are leaving the muzzle at different times despite having similar muzzle velocity values the reason their POI differs is because of different exit times. And BC differences also contribute. The latter being one of the reasons even a well-tuned rifle doesn't make super tiny ragged holes 100% of the time. Muzzle exit times differ. And bullets differ. We can only do so much to mitigate all the varying factors. And thus, POI isn't always what we think it should be. The more control we have over everything, the more sense the POI will make.

Just because the mv’s can be the same, it does not represent how it got there. I am sure barrel vibration would be different, and has the potential to change poi. In centerfire, two different powders with the same velocity and the same components are not likely to have the same poi. I don’t think rimfire can be made perfect enough to have common mv as the only contributing factor to poi shift.

As I understand it, barrel vibrations are pretty similar no matter the ammo, as they're largely dictated by the physical characteristics of the gun and everything attached to it. The magnitudes of what is going on can differ depending on the ammo, but the overall behaviour stays mostly the same. Different primers and powders affect muzzle exit time, so we see the same effects there resulting in different POI. How much energy there is in a given round can affect the magnitude of what goes on, but the what goes on part is more or less dictated by the physical characteristics of the gun. Barrel contour and length, mass of the gun overall and its centre of gravity, these are things that determine what that barrel is going to do during firing. There's even at least one calculator available on the internet that can show you what the basic behaviour of the barrel will be based on those things, including the overall mass of the rifle and how far above/below the bore the CoG is, which will be affected by the scope and stock, etc. And in case you miss it while glancing at the calculator, it is entirely missing anything to do with your ammo or load because it doesn't really matter here. And I think that's precisely because the ammo only changes the magnitude of what's going on. The variables you can enter into that calculator are what's responsible for the general behaviour, not the load.

 

[_Shorty]

I am just being a science nerd here, but that statement above is enough to render the investigation invalid (Re pulling bullets and measuring components, etc.). The primer and powder are THE energy source which accounts for all of the kinetic energy and resultant MV. If the assumption is that the weighing apparatus was not fine enough to measure true powder differences to the required significant number of grain decimal points, then we have a problem.

If one was testing for bullet weight variance, then the powder charge variable must be controlled for statistically. Sounds like a laboratory quality weigh scale is needed for the powder, like a Sartorius that measures to the 0.001 grams (0.0154 grains), or maybe to 0.0001 grams (0.0015 grains), accurately. How many decimal points are necessary? I don't know, but that can be experimentally determined.

Quantifying the difference in chemical energy load between every cartridge, to me sounds like Job 1 to develop the methodology for.

I'm not sure I get the point of what you're saying here. How is it invalid because one of the components seemed to have a variance smaller than the scale's resolution? I think the point I was making was that bullet weight varies quite a bit. Having trouble seeing a difference in the powder charge weight due to the equipment used doesn't change that. Bullet weight still varies a lot even if I don't know how much the powder varies.

 

[_Shorty]

Regarding the first sentence, there may be less sense. Of course shooters can't measure bullet velocity as it travels through the bore. But that's not necessary. With two .22LR rounds at the same MV, there's only one way can they go from 0 fps at the chamber/leade to their identical muzzle velocity some 24 - 26 or so inches away. It's not possible for the rate of acceleration in the same bore to be different for two rounds with the same MV.

This is not correct, and I do not know why you would think this is the case. Having the same muzzle velocity does not mean they must have had the same acceleration curve. Ever watch drag racing? Two cars can have the same elapsed time but have different trap speeds because one had a different acceleration curve than the other. Two cars can also have the same trap speeds but different elapsed times because one had a different acceleration curve than the other. What we're talking about here with bullets exiting the muzzle is no different. Two bullets can leave the muzzle at 1085 fps but have different acceleration curves. Different amounts of pressure at any given moment in time result in a different amount of acceleration at any given moment in time. Pressure traces like for the seven shots shown below are really wiggly because the powder isn't burning at a steady rate, and the result of the varying burn rate is a varying amount of pressure, and this results in a varying amount of bullet acceleration. You can have one shot burn hotter early but run out of powder sooner and have it exit the muzzle with a velocity the same as another shot that burns cooler but has more powder to burn longer. They will have the same muzzle velocity, but will have different acceleration curves, and this results in a difference in muzzle exit time. Muzzle exit time isn't directly tied to muzzle velocity, and I think this is a view you currently hold. But that view is incorrect.

from https://www.frfrogspad.com/intballi.htm

 

[grauhanen]

I don't know what ammo he was weighing, but nothing I've weighed had bullets that varied so little. The brick of Eley Club that I measured the other day looks like this:


I've measured a lot (many 5000-round cases) of ammo, and I've never seen anything with bullet weight variance as low as that guy's talking about, so I am afraid I call BS on that one. I have three muffin tins that I use for sorting, and they're half a dozen bins each, and are labelled in 0.1-grain increments. It is unusual if I have more than a handful that don't fit in those 18 bins, as was the case with this lot of Eley Club. It needed 20 bins, but barely. Most of the time those 18 bins are enough, and a small number of them are likely to go unused. Never have I ever measured a brick or case of ammo and only used 2-4 bins, which would be the case if I saw similar results to that dude. Usually ~15 bins is enough, and it doesn't go over 18 often.



You don't need to know whether or not their acceleration curves were different. You only need to understand that it is possible to have different acceleration curves and still arrive at the same muzzle velocity. But you do not seem to be able to fathom that this is possible for some reason. Just like it is possible for two different cars to both cross the finish line in a 1/4-mile drag race at 150 mph but have very different elapsed times it is also possible for two bullets to have the same muzzle velocity but take different amounts of time to exit the muzzle. You still don't seem to be understanding barrel harmonics properly with this statement. Barrel vibration patterns aren't random. They're actually fairly consistent. What's not very consistent is when bullets exit the muzzle in relation to their vibration pattern. When your bullets are leaving the muzzle at different times despite having similar muzzle velocity values the reason their POI differs is because of different exit times. And BC differences also contribute. The latter being one of the reasons even a well-tuned rifle doesn't make super tiny ragged holes 100% of the time. Muzzle exit times differ. And bullets differ. We can only do so much to mitigate all the varying factors. And thus, POI isn't always what we think it should be. The more control we have over everything, the more sense the POI will make.

With regard to bullet weight, it's important not to confuse cartridge weight with that of the bullet. Unless a bullet is disassembled (and destroyed) the weight of the bullet remains undetermined. In any case, small variations in bullet weight do not account for the egregious mismatches in MV/POI that is the weird thing in this thread.

When bullets leave the muzzle with the same MV, their "acceleration curves" must remain the same within the same bore. There's only one way for a bullet to get from 0 fps to an MV of say, 1085 fps in the same bore. Perhaps different acceleration rates may be possible with different bores that have different dimensions and characteristics. But that's not what's being discussed here.

Accepting that barrel vibrations are not random, they are propagated by the same forces with each shot. For simplicity, within the same barrel they are the force of the firing pin striking the cartridge rim, the detonation of the primer and the powder, and the accelerating bullet (up to a point, depending on barrel length). Rounds with the same MV can be expected to generate the same forces to cause similar barrel vibration. Of course different barrels may experience different vibration patterns with rounds of the same MV but that's because they are different barrels.

 

[grauhanen]

This is not correct, and I do not know why you would think this is the case. Having the same muzzle velocity does not mean they must have had the same acceleration curve. Ever watch drag racing? Two cars can have the same elapsed time but have different trap speeds because one had a different acceleration curve than the other. Two cars can also have the same trap speeds but different elapsed times because one had a different acceleration curve than the other. What we're talking about here with bullets exiting the muzzle is no different. Two bullets can leave the muzzle at 1085 fps but have different acceleration curves. Different amounts of pressure at any given moment in time result in a different amount of acceleration at any given moment in time. Pressure traces like for the seven shots shown below are really wiggly because the powder isn't burning at a steady rate, and the result of the varying burn rate is a varying amount of pressure, and this results in a varying amount of bullet acceleration. You can have one shot burn hotter early but run out of powder sooner and have it exit the muzzle with a velocity the same as another shot that burns cooler but has more powder to burn longer. They will have the same muzzle velocity, but will have different acceleration curves, and this results in a difference in muzzle exit time. Muzzle exit time isn't directly tied to muzzle velocity, and I think this is a view you currently hold. But that view is incorrect.

Shorty, you're clinging to a discreditable argument. In the same bore, when a bullet begins at 0 fps and accelerates to 1085 fps at the muzzle it gets there in one way only. Two .22LR bullets in the same bore will not accelerate in different ways to get to the same MV.

While different "acceleration curves" provides a convenient explanation to support and lend credence to the view that bullets with the same MV may not cause the barrel to vibrate the same way, it's not one that holds water. It implies that a tuner does its job by causing allegedly different acceleration curves to change and suddenly become similar. It's like having the cake and eating it too.

 

[_Shorty]

Not sure if I kept all the data from back when I looked at this or not, but I did stumble upon one 10-round examination.

Looks like this was some CCI Standard back in 2020. So there might have been +/- 0.1 grains in the combination of the case, primer, and powder after removing the bullet, going by those 10 rounds anyway. And as I say, I couldn't see any difference in the powder itself because they all just read 1.0 grains. But if you look at the weight of the case+primer+powder and compare it to the weight of the case+primer after the powder was dumped out you can kind of see that there's probably some variance going on in the powder that's likely just below the 0.05-grain margin to make the scale tip one way or the other. But if there's pretty much just +/- 0.1 grains of variance in the case/primer/powder combination then that would seem to indicate that the rest of any loaded round variance is in the bullet. See a variance of +/- 2 grains in a lot like the Eley Club shared previously and probably +/- 1.9 grains of that is in the bullet. So, no, you don't really know how much of the total variance is due to any single component because you're weighing all components at the same time when weighing a loaded round, but if you tear some apart to check you should find that the vast majority of it is going to be in the bullet. And after doing that with a few different brands/lines I just now assume that most of it is in the bullet, since that's what all indications suggest. (The rims were probably all 38.0 thousandths because they may have been pulled from some that had already been sorted by rims.)