Can you reverse engineer velocity somehow?
- Thread starter greg11
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I had a calculator once that did that but you needed almost every other variable and it was still just an estimate , damned if I could find it, maybe hunt google for an hour or two.
Avalonthas
CGN Ultra frequent flyer
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Ben hit it on the nail
Bringing my mind back to Grade 11 physics class hehe.....Velocity = Distance / Time
Hotwheels81
CGN Ultra frequent flyer
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the sinic would say your stupid for asking that question
the optimist would try to help you figure it out
the handloader in me is asking, Why is it important for you to know your velocity?
the optimist would try to help you figure it out
the handloader in me is asking, Why is it important for you to know your velocity?
1) yes, if you know the drop and the bc you can keep trying velocities in a ballistics calculator until you match the drop. But I expect it would be not terribly accurate because of uncertainties in drop (are you good enough to put several groups at the same POI?), and inaccuracies in published BC numbers (Sierra and Lapua are often closer than other brands) - you could compensate by buying Litz' book and using his measured BCs.
2) no... because a usable chronograph can be had for about $100 (Shooting Chrony F-1). Better ones for about double (CED M2).
It just depends on how accurate you need the velocity to be. If you do proceed, you should think about using "OnTarget Precision Calculator" to measure drop from the true group centre.
??? - most handloaders want to know that. Pretty basic to the game - especially if a hunter. Also measured velocity of a given load can help to satisify one that pressures are reasonable.
But imagining and foggy guessing - sorrry, "reverse engineering somehow" - don't cut it IMO - for the handloader. Results will be so imprecise they will not be very meaningful. Look into getting a reliable chrony as mentioned already above and know velocity for sure - IMO. Mine was under $100 and have been using it almost 30 years. Then play with bullet drop and known ranges to guess (calculate) if manufacturers BCs are optimistic - which they often have been.
My 2 cents - don't ask for your change back.
While, in theory, it should be possible, I am also in the camp that says buy a chronograph. The percentage of error in your calculations will be so high with the "reverse engineered" versions that it would be useless, and chronographs are really quite cheap in terms of reloading and shooting accessories. There is no point in spending a lot of time and effort getting information you can't trust anyway.
Nonsense. With good BC data on the bullet and accurate drop measurements over several different ranges (especially out to 600yds, as OP states) it is quite possible to use a ballistics calculator to figure out the muzzle velocity within 50fps at least, probably better.
Now, why OP wants muzzle velocity as opposed to simply measuring the come ups on his 600yd range is a different question. Myself, I would rather have the actual measured drop data than the velocity.
Mark
And how good is the BC data? That is a variable, and a complex one at that, if you do some research.
How good is the drop measurement? Another variable.
How good is range estimation? Another variable.
Add them all up, and you will get a serious error percentage. For example, the difference between point of impact at 600 yards for a .30 cal. 180 gr. Ballistic Tip changes by about 7 inches (Nosler manual) for an initial velocity difference of 100 fps. A 50 fps velocity difference would make about a 3.5 inch difference in POI at 600 yards.
Do you really think you have the ability to accurately measure that difference at 600 yards? Any rifle that will shoot a group smaller than that difference, so you can actually measure it, is a VERY good rifle in deed. Many will not be able to shoot a group smaller than the difference in POI, which would make the error in measurement even more gross. Add in the other sources of error and the whole thing gets rather unscientific.
I repeat. In theory, it is possible. In actual practice, it gets very unlikely to be worth the effort for bad information. My advice is to buy a chronograph.
There's often a problem with that first important step. When chronographs became commonplace some bullet manufacturers reduced their BCs - which were optimistic often - some still are IMO. Also BCs are not just dependent on bullet shape. Atmospheric conditions and other things like stabilization in flight aside, Sierra for example gives often different BCs for the same bullet - dependent on velocity. But since velocity is the unknown here and what is sought, you may not have a good firm starting point for calculations.
It would be an interesting test though - to give someone loaded rounds, a 600yd range and a manufacturer's BC. When the figuring was over then check results with a chrony. Be fun to try.
I take my bullet BC data from Bryan Litz's work, the manufacturer data is very suspect as you mention. Since OP is looking for ballpark numbers, it isn't really that important for the drop data to be perfect. With your 180gr .308 ballistic tip and a 100yd zero, the drop data at 600yds is:
2600fps - 105.2"
2700fps - 96.2"
2800fps - 88.3"
Each 100fps equates to roughly an 8" change in drop. A 1MOA rifle (at 100yds) should shoot ~7-8" groups at 600yds on a calm day. I think that OP should be able to measure a group center to within 4% on 100" to get within 50fps of an accurate velocity measurement.
As noted, Bryan Litz's BC data is very good and uses the G7 reference instead of the G1 to minimize the velocity dependence of the results.
All that said, OP is way better off to simply drop the $200 on a chrony and do it the much, much easier way.
Mark
And use a laser level with some survey sticks to determine if your ground is level. Over 600yrds it would be very difficult to measure if the ground was level and those "few" inches will make a big difference.
I am in the camp that asks "Why?" MV, other than being a useless bit of trivia, is useful only in predicting projectile trajectory. (One assumes the OP is not trying to sneak an entirely inappropriate cartidge in to qualify in some game department's "minimum energy" requirements.)
If the OP chooses to actually measure his bullet drop, especially if he tracks his trajectory over several data points - 100m 200m 3, 4, 5 and 600m, for instance, he will have a true trajectory, and not the one guesstimated using some ballistic calculator based on theoretical BC's, standard atmospheres, one specified distance from bore to line of sight, yadda yadda yadda.
Of course, if the OP is fixated on knowing his MV, plus or minus some instrument arror, he should go out and purchase an Oehler, and measure it. He will still need to work up bullet drop using observed trajectory, but that is just good fun. It is called shooting.
If the OP chooses to actually measure his bullet drop, especially if he tracks his trajectory over several data points - 100m 200m 3, 4, 5 and 600m, for instance, he will have a true trajectory, and not the one guesstimated using some ballistic calculator based on theoretical BC's, standard atmospheres, one specified distance from bore to line of sight, yadda yadda yadda.
Of course, if the OP is fixated on knowing his MV, plus or minus some instrument arror, he should go out and purchase an Oehler, and measure it. He will still need to work up bullet drop using observed trajectory, but that is just good fun. It is called shooting.
So why not buy a Chromo?
While watching an individual use a chrono with one set of screens at 3 feet and getting a second reading through 5 feet. Since there was a difference on 12 fps on the two readings, he determined a loss of 6 fps. Therefore, at 300 feet, according to his calculations the bullet would lose 1800 feet per second in 100 yards.
now if the bullet started out at 3000 fps it would be coming back before it got to 200 yards . . . Yeah right . . . and chickens have teeth!
The BS that some people will spread!
While watching an individual use a chrono with one set of screens at 3 feet and getting a second reading through 5 feet. Since there was a difference on 12 fps on the two readings, he determined a loss of 6 fps. Therefore, at 300 feet, according to his calculations the bullet would lose 1800 feet per second in 100 yards.
now if the bullet started out at 3000 fps it would be coming back before it got to 200 yards . . . Yeah right . . . and chickens have teeth!
The BS that some people will spread!
Many people have grea results using hand held ballistic programs. By entering accurate information (velocity, bc or variable bc's, atmospheric conditions) and they get accurate drop charts. Well I would have to say that if you entered in accurate drops, weather conditions and bc's you would end up with just as accurate a velocity as the former would get in drops.
Somewhere around I have my scribblings of a loadouts 100y poi, 200y poi and 300y poi. I reverse engineered it to 3263fps. I chrono'ed it and it said 3300.
Somewhere around I have my scribblings of a loadouts 100y poi, 200y poi and 300y poi. I reverse engineered it to 3263fps. I chrono'ed it and it said 3300.
