Chamber pressures and bullet seating

[New Camper]

A question that has been on my mind for some time: Most reloaders state that a round with bullet seated shallow in the case, so that when chambered will almost touch/touch the rifling, will produce higher chamber pressures when fired. I cannot figure out the physics of the situation, for when compared to a factory load where the bullet will have to travel 100ths, even 10ths of an inch before engaging the rifling, it is accepted the pressure is less than the previous scenario. In first scenario, there is more air space in the case, and immediate acceleration of the bullet is in the rifling, which of course is where most friction is, however it is consistently being accelerated by the burning powder whereas in second scenario(factory load) the bullet travels freely, then slams into rifling: Initially there is less case "capacity" because the bullet is seated into that "capacity"(when capacity is reduced, pressure increases) Although the bullet will travel the tenths of an inch, to ameliorate that pressure increase, it then slams into the rifling(most friction here, so in theory, the acceleration will momentarily be reduced compared to the open space it just travelled) Furthermore, when you consider Newton's laws, the bullet has not had a spin imparted on it, so is travelling several hundred feet per second in a straight line, before engaging the rifling, then spinning(visualize the wheels of an airplane on touchdown 0mph to 50mph depending on stall/touchdown speed of the particular plane) Again, contacting the rifling where the most friction is encountered would then not produce the same believed pressure spike?

From my limited physical science experience, I am not inclined to accept the status quo and of course I have no way of proving or disproving. It is of course inconsequential unless someone is dabbling with very heavy loads, pushing the boundaries of pressure to new heights: Something that I and most people are not inclined to do.

Anyone have data on this subject? I am even open to the theorizing of others.

On a less intense and pressing subject: The 6mm Remington being a well balanced cartridge I think based on (necked down alone?) the very fine 257 Roberts. Has someone already necked these medium length cartridges up to 6.5 mm (264 thousandths of an inch) to take advantage of the myriad of bullets available for that caliber?

Additional consideration for why someone might conceive of such a wildcat: It seems logical to have a cartridge with case capacity somewhere between the 6.5-06(long case for that caliber) and the short actions in 6.5/260 derived from 308 cases? The 257 case length measures between the long and short mentioned, however the 30-06 parent would have some inefficiencies due to all the empty case space, and the short cases would suffer a lack of capacity, and of course resulting chamber pressure would increase +/- 20% over the former. Beating a pointlessly killed horse?

 

[H4831]

You said---"I have no way of proving or disproving."

Welcome to the club!
So many, many of these theories on shooting, reloading, etc, are just that--theories, and often can't be proven or disproven.
But they sure make for lively discussions!

I get a kick out of people explaining how a bullet starts out, then hits the lands, hesitates, then gets sized, etc, etc etc.
Here is a truism, not a theory.
The average bullet from an average bottle neck rifle case, reaches the speed of sound, by the time it has travelled 1.5 inches down the barrel.
How does this fit into your hesitating starts and moving motions as it gets to the lands?
A long time ago, when I was taking the training to be a flight instructor, I was taught to never tell a student any theory, or anything about flying, unless I could demonstrate and prove it to him/her in flight.
In other words, if a theory can't be proven, it is a nothing.

 

[Ganderite]

When the primer ignites the powder, the pressure curve is very steep. What prevents the gun from becoming a pipe bomb is the bullet pops out and starts down the barrel, increasing the volume of the container.

When does the bullet start to move? if the bullet is not touching the rifling, it starts almost immediately. In fact, the primer alone (if there is powder in the case) will start the bullet moving.

If the bullet is already stuck in the rifling, the pressure must build quite a bit higher before it can engrave the bullet and start it moving. Depending on the throat size and shape, touching the rifling adds about 5,000 psi to the pressure. This is very easy to measure with a pressure gun.

If you load ammo that is a bit too hot (pops primers) you can try seating the bullet 30 thou deeper to reduce pressures. The other trick (I have used it several times) is to smear a bit of grease on the bullet ogive. This drops pressures about 5,000 psi.

 

[22to45]

When the primer ignites the powder, the pressure curve is very steep. What prevents the gun from becoming a pipe bomb is the bullet pops out and starts down the barrel, increasing the volume of the container.

When does the bullet start to move? if the bullet is not touching the rifling, it starts almost immediately. In fact, the primer alone (if there is powder in the case) will start the bullet moving.

If the bullet is already stuck in the rifling, the pressure must build quite a bit higher before it can engrave the bullet and start it moving. Depending on the throat size and shape, touching the rifling adds about 5,000 psi to the pressure. This is very easy to measure with a pressure gun.

If you load ammo that is a bit too hot (pops primers) you can try seating the bullet 30 thou deeper to reduce pressures. The other trick (I have used it several times) is to smear a bit of grease on the bullet ogive. This drops pressures about 5,000 psi.

I presume that you have seen this test performed, to have the numbers on hand. I know that moly coating bullets drops the velocity, which must be because of the lowered pressure. Did your greased bullets go slower?

 

[Neilm]

All smokeless powders burn faster when confined and the higher the pressure the faster the burn rate. Put a small measure of any poeder on a open tray and light it and it fizzles. That is why a bullet touching the lands has a higher pressure. It takes more pressure to get it started and consequently the the remaining powder is burning more rapidly .
Moly bullets with the same charge usually have a lowere velocity because there is not as much resistance to the bullet going through the barrel so the powder burns slightly slower. If using Moly bullets you should use either a faster burning powder or an increased charge of the same powder. Ib either case proceed with caution.
The 6.5 257 has been around for years in the European 6.5 X 57. An excellent round but you should use a long action and then it loses to the 25-06.

Neil

 

[warrenb]

I've had different experiences seating into the lands. I recently started working up loads for 338 LM. I started 10 thou in the lands. 83 being the start load and 92 being the max of the powder I was using. At 85 grains I was getting lightly raised rings in the case head from brass flow and slightly stiffer bolt lift. The accuracy indicated wild pressure spikes. I pulled bullets and reloaded 10 thou off the lands and I'm now shooting a charge of 90 grains with no pressure signs, good but not excessive velocity for 300 grains at 2700 fps and a pretty good SD of 6.9.

Conversley I have a 223 which I load with light neck tension. I stuff the rounds 25 thou into the lands. I use a charge considerably over the max published and I have no pressure signs. Again velocity is good but not excessive for 80 grains at 2750. The ES is fairly tight in mid 20s typically. No indications of pressure spikes.

From my sample of two. I would say its best to err on the side of caution when seating into the lands and expect pressure spikes.

 

[Andy]

And just so it's "clear":

- if you seat the bullet long, so it's touching or very near the rifling, the pressure will be higher; and
- if you seat the bullet shorter, away from the rifling, the pressure will also be higher, because you are reducing effective cartridge capacity.

Many believe that there is a magic place about 0.030" from the rifling where the pressure will be lowest. Maybe it is, but I choose the OAL where the load is most accurate. We never know the precise pressure on any given day with any given powder lot, but we do know that rifles are built so that a load won't be perfectly safe at 0.030" off the lands, and unsafe 30 thou longer.

As for "pressure signs" - it's a very personal thing. There are many explanations for these subjective observations, and you can't assume that a load is under a certain pressure because you don't see them, nor above when you do.

 

[New Camper]

Wow, nobody called me an imbecile or told me to stop reloading: All replies from gentlemen!

I am grateful for the replies despite the few variances. One thing I cannot figure is the throat of a rifle is distance X: How can the pressure (aside from the obvious spike right at ignition) be higher or lower when the bullet has to travel X before engaging the lands regardless of whether the bullet is seated in X or has to travel X?

I agree that a primer alone is capable of putting a bullet into the rifling: I shot a reloaded shotshell years ago that just popped: Everything came out of the barrel, granted a rifle barrel despite having no choke, would still produce far more friction, but is a lighter load.

When considering such things, a couple phrases come to mind: Let's call them universal principles: When in doubt, don't do it and assumption is the mother of all b{beep}es.

I have only overloaded a few, and whether it's the primers I don't know, but I invoked one of the universal principles: I had seated to the rifling, a near max load and the primer sprung a leak at the impact. Safety glasses prevented gas and particles from hurting my eyes, and even if it was heavy enough, the specks wouldn't have made me uglier. This would support the theory that bullets contacting the lands create higher chamber pressures. Of course, I didn't replicate the same load (despite being below maximum listed in manual) with the bullet seated deeper. Interestingly it was still an MOA load.

Is there a publication that focuses on pressure testing laboratory findings? It stands to reason that a company like Speer that has been around for decades, testing, would have collated all their findings into a manual. Of course "welcome to the nineties" and the liability issues of people doing things beyond a companies control might discourage such publications.

I get a kick out of people explaining how a bullet starts out, then hits the lands, hesitates, then gets sized, etc, etc etc.
Here is a truism, not a theory.
The average bullet from an average bottle neck rifle case, reaches the speed of sound, by the time it has travelled 1.5 inches down the barrel.
How does this fit into your hesitating starts and moving motions as it gets to the lands?

Was my explanation logical? Or just amusing? I did acknowledge that in the short distance (a fraction of the 1.5" you state being the distance a bullet reaches +/-1000fps) the bullet would be travelling several hundred feet per second, but as another poster reaffirmed, the case neck provides little friction, and the most is encountered at the rifling. True, there isn't a literal reduction in speed, but I was suggesting the contact with the rifling would be where the bullet would be accelerated the slowest, regardless of whether it starts at the rifling or takes a few tenths of an inch trip before contact. In the fractions of a second, I doubt we will ever be able to measure what happens in the chamber, but bringing it to a situation we can easily see like pushing a car: It rolls fine on the street with lots of pushing, but if you shove it between the neighbours cars, that are parked a couple thousandths of an inch closer than your car is wide, one man won't be able to push it. Sorry, that's a silly example.

The other theory that I was postulating in the bullet travelling the throat before rifling contact: It will be travelling a few tenths of an inch within a spin being imparted on it, a few hundred feet per second. Being a flight instructor, you have probably watched the wheel on a C172 on touchdown: The wheels will always chirp as they are accelerated from 0mph to(approximately as stall speed, pilot control and environment can cause variations by several mph) 55mph. I use this comparison to illustrate what happens(my theory) to the bullet travelling several hundred feet per second, as it contacts rifling. My flight instructor never mentioned unproven theories, but in the more relaxing flights like spins and spiral dives, we had many laughs. I am told I was his only student to, "crop dust"... Probably because I was the only one dumb enough to book six hours of lessons in one day, at the time when the upside down or multiple g experiences would be encountered.

Another thought occured to me: Yes, we can with varying degrees of interest and pleasure discuss this subject: Are we in danger of becoming like the ancient Greeks, as they discussed how many spirits could dance on the tip of a pin? Firearms philosophers!

 

[grouch]

The 6.5/257 was done many years ago. I imagine the reason it never went anywhere was that 6.5x55 already fills that slot.
grouch

 

[Ganderite]

In a rifle, seating the bullet deeper does not increase pressures. At least, not that I have been able to notice, and I have fired several hundred thousand rounds through pressure guns in various ballistics labs. You might note that as you go from a loose charge to a mildly compressed charge and a heavily compressed charge, the pressure increases are linear. In a rifle case, air space is not a big issue, unless there is too much.

In a pistol it can make a difference in some short cases like 9mm, 40 S&W and 45ACP.

Neck tension is a much bigger variable than the average shooter gives it credit for.

It can be used to boost pressure (for a more uniform burn). Here is a simple experiment every rifle re-loader should try:

Size and load 10 cases the usual way.

Take another 10 sized cases and remove the decapper/expander rod and re-size, so that the neck is squeezed down and not opened with the expander button. This will give you max neck tension. Load these cases the same as the other ten.

Shoot these rounds for both velocity (and SD) and for accuracy. If the tight necks shoot better, remove the expander button on you sizing die, or use the rod from a smaller caliber.

Accuracy:

There are two variables at play that are somewhat linked - velocity and pressure.

At a certain velocity the barrel will perform best and shoot the best groups. Let's say it is 2800 fps.

At a certain pressure(typically near max pressure) the powder will burn the best and produce the best SD. You may have noted that the best accuracy load is not always the same as the best SD load. The best SD load may produce more veocity than you want. How to keep the lower velocity but get higher pressures?

Higher velocity relates to higher pressure when the higher pressure is cause by more powder - the usual situation. But there are other ways to boost pressures:

Seat the bullet into the rifling. This will boost pressure but have a small effect on velocity.

Increase neck tension.

Switch to a non-coated bullet.

Conversely, if you want to keep a larger powder charge, but reduce presures:

Back off the rifling to about 20 though off.

Switch to a coated bullet.

Decrease neck tension.


You will find one day that it is hot and the ammo you developed in cooler weather in now popping primers or causing stiff extraction. How to reduce pressure on the spot for this ammo?

You or someone else will have some grease in his kit used for lubing locking lugs. Smear a thin film of this on the bullet ogive. It drops pressure about 5,000 psi and does not hurt accuracy. I have tried it a few times and it seemed to improve accuracy in my limited tests.

 

[Andy]

Now that is interesting. You will find many instances where rifle shooters are warned not to go below a min OAL, possibly because powder manufacturers specify it. If not for safety, then why do they state a min OAL?

Here's a good article that says what Ganderite said, but in many more words and with pictures: http://www.hornady.com/ballistics-resource/internal

This is just a pic from the web, but if Ganderite and Hornady are correct, it illustrates it pretty well:

All that having been said, I still load for accuracy, not lowest pressure. That having been said, accuracy seems to be best "just off the lands", not into them, and there are other reasons to keep a bullet away from the rifling, such as doing away with the problem of a bullet being unseated if it needs to be unchambered.

In a rifle, seating the bullet deeper does not increase pressures. At least, not that I have been able to notice, and I have fired several hundred thousand rounds through pressure guns in various ballistics labs. You might note that as you go from a loose charge to a mildly compressed charge and a heavily compressed charge, the pressure increases are linear. In a rifle case, air space is not a big issue, unless there is too much.

In a pistol it can make a difference in some short cases like 9mm, 40 S&W and 45ACP.

Neck tension is a much bigger variable than the average shooter gives it credit for.

It can be used to boost pressure (for a more uniform burn). Here is a simple experiment every rifle re-loader should try:

Size and load 10 cases the usual way.

Take another 10 sized cases and remove the decapper/expander rod and re-size, so that the neck is squeezed down and not opened with the expander button. This will give you max neck tension. Load these cases the same as the other ten.

Shoot these rounds for both velocity (and SD) and for accuracy. If the tight necks shoot better, remove the expander button on you sizing die, or use the rod from a smaller caliber.

Accuracy:

There are two variables at play that are somewhat linked - velocity and pressure.

At a certain velocity the barrel will perform best and shoot the best groups. Let's say it is 2800 fps.

At a certain pressure(typically near max pressure) the powder will burn the best and produce the best SD. You may have noted that the best accuracy load is not always the same as the best SD load. The best SD load may produce more veocity than you want. How to keep the lower velocity but get higher pressures?

Higher velocity relates to higher pressure when the higher pressure is cause by more powder - the usual situation. But there are other ways to boost pressures:

Seat the bullet into the rifling. This will boost pressure but have a small effect on velocity.

Increase neck tension.

Switch to a non-coated bullet.

Conversely, if you want to keep a larger powder charge, but reduce presures:

Back off the rifling to about 20 though off.

Switch to a coated bullet.

Decrease neck tension.


You will find one day that it is hot and the ammo you developed in cooler weather in now popping primers or causing stiff extraction. How to reduce pressure on the spot for this ammo?

You or someone else will have some grease in his kit used for lubing locking lugs. Smear a thin film of this on the bullet ogive. It drops pressure about 5,000 psi and does not hurt accuracy. I have tried it a few times and it seemed to improve accuracy in my limited tests.

 

[New Camper]

It appears then the "magical" sweet spot(when pressure is the consideration) of thirty thousandths of an inch off the lands is correct: That Hornady link was simple in explaining some of what I wished to learn.

When I handload wadcutters for my 357mag, I put a slight crimp to contain the bullet longer, for more thorough ignition/consistency. It sounds funny to handload 357mag wadcutters, for it seems to defeat the purpose of having that over a 38special. This amount of OCD doesn't mean much to my accuracy, as my abilities with handgun pale in comparison to competitors yet. This thought should have occured to me as there are similarities to rifle rounds. So as it requires less energy to keep something moving than to accelerate it, so is the relationship with the bullet and gap to rifling subject.

I digress: Reading and learning!

 

[warrenb]

Very informative post Ganderite.

When I first started reloading I would use the Lee Factory crimp die on all my rounds. I stopped using it as I felt it was not easy to be consistant with neck tension. It was only later that I put 2 and 2 together. I realised I was able to increase my charges before hitting my predetermined max charge when the rounds had less neck tension.

I had always surmised that additional pressure (eg. more neck tension or round in rifling) would give an increase in velocity.

 

[350 Mag]

And just so it's "clear":

- if you seat the bullet long, so it's touching or very near the rifling, the pressure will be higher; and
- if you seat the bullet shorter, away from the rifling, the pressure will also be higher, because you are reducing effective cartridge capacity.

Many believe that there is a magic place about 0.030" from the rifling where the pressure will be lowest. Maybe it is, but I choose the OAL where the load is most accurate. We never know the precise pressure on any given day with any given powder lot, but we do know that rifles are built so that a load won't be perfectly safe at 0.030" off the lands, and unsafe 30 thou longer.

As for "pressure signs" - it's a very personal thing. There are many explanations for these subjective observations, and you can't assume that a load is under a certain pressure because you don't see them, nor above when you do.

Yep, lots of guys think a flattened primer is over-pressure.

I follow recipies in reloading guides pretty close, and IF you know someone with QuickLoad that is a invaluable tool to project expected pressures and to give you an idea when things can start to become dangerous.

I like to look at lots of things.

Primers. specifically looking for cratered primers.

Sticky extraction.

Marks on case head left by bolt face or extracter

Velocity exceeding book values.

Miking case head expansion.

 

[Lead Loader]

i would suggest go on the hornaday or winchester website the one has internal ballistics which is a good read.

 

[SandRoad]

It appears then the "magical" sweet spot(when pressure is the consideration) of thirty thousandths of an inch off the lands is correct: That Hornady link was simple in explaining some of what I wished to learn.

When I handload wadcutters for my 357mag, I put a slight crimp to contain the bullet longer, for more thorough ignition/consistency. It sounds funny to handload 357mag wadcutters, for it seems to defeat the purpose of having that over a 38special. This amount of OCD doesn't mean much to my accuracy, as my abilities with handgun pale in comparison to competitors yet. This thought should have occured to me as there are similarities to rifle rounds. So as it requires less energy to keep something moving than to accelerate it, so is the relationship with the bullet and gap to rifling subject.

I digress: Reading and learning!

One thing is to not confuse large capacity, bottleneck cases, with low capacity straight wall cases. Setting back a 9mm or 40 S&W will substantially increase pressure. Why? Because peak pressure on these handgun cases occurs before the base of the bullet even clears the case mouth.

In typical rifle cases peak pressure is an inch or so down the barrel. Magnum overbore cartridges hit peak pressure one to two inches down the barrel.

 

[H4831]

One thing is to not confuse large capacity, bottleneck cases, with low capacity straight wall cases. Setting back a 9mm or 40 S&W will substantially increase pressure. Why? Because peak pressure on these handgun cases occurs before the base of the bullet even clears the case mouth.

In typical rifle cases peak pressure is an inch or so down the barrel. Magnum overbore cartridges hit peak pressure one to two inches down the barrel.

This is exactly why I don't get concerned about extra pressure when seating the bullets to the lands in bottle neck rifle rounds.
If there is a spike in pressure, it is well before peak pressure is reached.

 

[sobo4303]

New Camper...there is one critical piece of information that you are assuming to be correct. You are assuming that the powder is instantaneously burned off at the moment of ignition.

Remember, this is all happening within a micro/nano second

Nope not correct. The moment the firing pin strikes the primer, the primer ignites. The primer flash ignites the powder and very rapidly the pressure builds to move that bullet. As the bullet moves the pressure lowers somewhat...but it the minor reduction is offset by the continuing burning of the powder. The bullet by this time, hits the lands and...by coincidence, the pressure is building to the maximum peak. This pressure is required to force the bullet into the rifling, which in turn, carves the grooves into the bullet. The powder, is still igniting and will continue to do so as the bullet travels down the barrel, always providing that force to propel the bullet forward. Keep in mind that the further the bullet travels down the barrel, the less friction it will encounter because the rifling has already imprinted into the bullet.

Remember the flash you see sometimes, that's a slower burn powder still burning, light, heat and plasma energy.

I seat my bullet for accuracy. Every rifle is a little different. In some cases, the magazine may limit the seating depth.

 

[colinmatchett]

"Remember the flash you see sometimes, that's a slower burn powder still burning, light, heat and plasma energy."

I thought the flash was the ignition of gases

 

[New Camper]

New Camper...there is one critical piece of information that you are assuming to be correct. You are assuming that the powder is instantaneously burned off at the moment of ignition.

Remember, this is all happening within a micro/nano second

Nope not correct. The moment the firing pin strikes the primer, the primer ignites. The primer flash ignites the powder and very rapidly the pressure builds to move that bullet. As the bullet moves the pressure lowers somewhat...but it the minor reduction is offset by the continuing burning of the powder. The bullet by this time, hits the lands and...by coincidence, the pressure is building to the maximum peak. This pressure is required to force the bullet into the rifling, which in turn, carves the grooves into the bullet. The powder, is still igniting and will continue to do so as the bullet travels down the barrel, always providing that force to propel the bullet forward. Keep in mind that the further the bullet travels down the barrel, the less friction it will encounter because the rifling has already imprinted into the bullet.

Remember the flash you see sometimes, that's a slower burn powder still burning, light, heat and plasma energy.

I seat my bullet for accuracy. Every rifle is a little different. In some cases, the magazine may limit the seating depth.

Yes, I understand that it doesn't all happen at once: To bring it into a scale we can readily relate to, it would require us to slow it down so that a micro second were like a minute. On this scale, the whole process from primer strike to bullet gone would be several minutes. It makes me think and really appreciate the precision of all these seemingly small things happening, that produce "only" a "loud bang". Lots going on, not unlike what is happening in one RPM of an engine. Scale...

My 222 Remington is somewhat indifferent to bullet position: It has decent groups with handloads OAL the same as factory, but has placed some decent groups with the bullet in the lands. My 270 Sako, however, seems to prefer the deeper seating of bullets... weird that an expensive gun would be picky.

I would attribute the flash not necessarily to slow burning powder because a shotgun or pistol leaves a nice flame trail in low light. Aside from the burning powder leaving the bore(under immense pressure), would it be logical to theorize that the oxygen quickly consumed within the chamber, would stifle burn, however the hot embers of the powder could finish burning after leaving the muzzle? A moot point, and irrelevant to the original question, but it does bring into consideration the chemistry of gunpowder.