Calculate muzzle velocity with drop.

[kilotanker22]

I have been pondering lately.

If we use Bullet data and muzzle velocity to calculate trajectory. The We could remove one known variable from that. Replace it with another and still solve for the missing variable. (pretty basic concept)

I have not had the motivation to go that route until now. (I Like to do things the lazy way..) Alas My Chronograph is broken, I have a new rifle that I have not yet tested. I am also saving up for a dopler radar setup.

I am gonna zero the rifle with factory ammunition. Use the factory ammunition data to calculate a zero. For the Ammunition I am using. Bullet velocity is 3100FPS. G1 BC is .61. The factory data would work out for me having a zero of 24 yards, This would also zero me at 300 yards. (for elevation that is)

Plug in the factory data for MOA adjustment at ranges from 24-500 yards.

Then shooting several groups at each of those ranges. Then using the Test data to compare to Factory. Using your test data (Actual trajectory) and bullet data. I intend to Calculate my actual muzzle velocity. Then re zero and confirm.

Then When I get That Radar. I can test my calculations against it. I know that this is totally doable. I am gonna do it, mostly because I am bored. And to start a conversation here. (When I nothing else to do.)

[doofus47]

Well, theoretically, you could do that, but you would have to dedicate a larger amount of ammo. If you knew an average MV per chrono using 10 shots, you could calculate the drop all along your flight path. If you don't know the MV, you need to fire X shots at each of your trajectory points 50, 100, 150, 200 yards, for example but you also need to fire several groups of bullets at each of those check points to average the impact points. Then you find the average of those to work out the velocity required to "end up" at those drops. If X is a larger number of shots, the average group size would be more "accurate" as figuring an average impact point.
If you like to shoot (and isn't that why we're all here) and you like to work out averages plus correctly accommodate the vagaries of your human intervention as the shooter at each target impact point, either way will work. But the easier way imho is to figure the average velocity of a few cartridges and then work out the math.

[Scorch]

With ballistic software, if you know MV and BC, you can calculate POI to within thousandths of an inch. One shot at each distance and you've verified the data.
Here's JBM bal;listics calculator, play with that one for a while. http://www.jbmballistics.com/

[Unconventional]

There are ballistic calculators where you can kind of do what you're looking for. It's called "truing". Best done at the furthest reaches of supersonic flight for your particular cartridge/load, you fire at a known distance target using the holdover/come up that the software predicts you should use. Fire at the target and adjust your elevation until you have POA-POI. Then, you tell the software what elevation was actually needed and it will spit out a muzzle velocity.

If you don't have access to a range where you can use the full range of supersonic flight of your cartridge, shorter distances will work. However, interpolation is probably better than extrapolation.

The software must know bullet weight, BC, atmospherics, and target distance.

Just as you mentioned in the original post, you can figure out a single unknown variable if you know the others. This same method can be used to get a rough estimate of an unknown BC of a particular bullet if you know the muzzle velocity instead.

It's been some time since I've used this method, so there is likely more accurate calculator software or methods.

I like your approach to doing this - let us know how it turns out.

[taylorce1]

How do you plan to compensate for human factor? If you're saving up for a Lab Radar you're going to burn a lot of money shooting enough groups that you can minimize the amount of human factor you put into your velocity figures. It's going to take a lot more than a couple of boxes of ammunition to remove shooter error from the equation, that's the main benefit of a chronograph is it gives you the info you need with the least amount of ammunition.

[jmr40]

Sounds like someone trying to drive from Atlanta to Dallas through Chicago. You'll get there, but are taking the long route.

[kilotanker22]

taylorce1, I know its gonna take a lot more ammo. Honestly to compensate for the human factor is gonna be an ongoing process. I intend to use averages of multiple groups fired at each range over many visits to the range. The more I shoot over time the more accurate my averages will be. Its more for curiosity and Boredom than anything else. For the time being until I have saved enough for a decent radar unit. I also load ammunition, so after i burn a few hundred rounds of factory ammunition (for brass) I will be loading and testing. I will probably have that Radar unit by then.

jmr40, I really like your analogy there.. Made me chuckle a little. I know its the long route, but it's nice to take the long way once in awhile. Changes the scenery, and challenges my shooting ability. And my understanding of ballistics.

[kilotanker22]

I guess I could always buy a Pro chrono or a like Chronograph, in the meantime. (While saving for the radar) But this will be interesting and Challenging. If I do buy another chronograph before I get the radar. It will probably be a Magneto speed. I have heard a lot of positive stuff about them. And I like the idea of the way it works as opposed to a light sensing chronograph.

[LeverGunFan]

Hats off to you for doing this experiment, I think that you (and we) will learn something from this. My guess is that the G1 BC is not constant over the velocity range that you will be shooting, so the calculated MV will be off some. How much will be an interesting result... Good luck and keep us informed of your progress!

[std7mag]

Yeah, it's been done.
Check out the Tube de You...

[taylorce1]

It's all about time and money, even a cheap $100 chronograph will save you a lot of both. You'll be making faster and more consistent hits down range by investing a little money upfront.

[Jim Watson]

I had an old Gun Digest article "The Trajectory Chronograph," that let you do that.
It had graphs and nomographs that solved for velocity without even a calculator, much less custom software on a computer.

[Mobuck]

I think that's the way some of the "old timers" determined the velocity of their "super dooper hand loads".

[FrankenMauser]

You need to calculate the drop based on bore axis, not the point of impact from another type of ammo (with its own variables and likely different ballistic coefficient).

If you try to calculate MV of an unknown load based on the zero from the MV, BC, and drop from a different load, you'll just be urinating into the wind.
No valid data will be produced.

[fourbore]

I believe this will absolutely work. The PROBLEM is you need an accurate BC.

This can be interesting. To be practical, if you measure the drop across your working ranges, what use is the velocity data? You have the answer that velocity is used to provide.

[kilotanker22]

Frankenmauser,

Each load tested will be zeroed prior to that test. I do see what you are saying though. I just want to see if I can get close and then compare calculated velocities across the board with actual velocities later on.

[std7mag]

Just as a curiosity...
If sighted in at 100, do drop at 300.
Would it be more "accurate" if done at say 600?
Or would BC start messing with the equasion if it were a few points off?

[kilotanker22]

I think it can be done reasonably accurately at 300. Definitely easier for the shooter to manage tight shots.
Would require about half the bullets. And if you shoot enough groups. At different times and range visits. Average your results. Then note how much your POI average deviates from the supplied data. I mean that's basically what you would be doing. At any of the ranges. Is recording D.O.P.E. Data On Previous Engagements. Getting to know that particular firearm and how it behaves in the real world. Based on the D.O.P.E that you collected over a period of time and situations you could pretty easily come up with a very close answer

[COSteve]

I can see a significant problem with this approach. That being the published MV using an unknown length test barrel, at an unknown altitude, at an unknown temperature, at an unknown humidity vs what your rifle, with your length barrel, at your altitude, at your temperature, at your humidity will actually produce.

I live in a dry climate at 6,100ft asl. The density of the air on a 'standard' day is 14.7lbs/sf at sea level, however, at 6,000ft, it's only 11.7lbs/sf. And the 'standard' temperature is different at different altitudes; at sea level it's 15° C but drops as you go higher so at 7,000ft it's on 1°C.

I learned to fly 50+ years ago and we had to learn all about density altitude. It affects more than just an airplane's engine and wings, it also affects the performance of ammo (MV) and the trajectories they produce.

[taylorce1]

Originally Posted by std7mag

Just as a curiosity...
If sighted in at 100, do drop at 300.
Would it be more "accurate" if done at say 600? 
Or would BC start messing with the equasion if it were a few points off?

The longer distance you can shoot it and measure the drop accurately, the better your data will be. However, if you haven't calibrated your scopes tracking all you're doing is introducing more error into your equation.

[kilotanker22]

Originally Posted by taylorce1

The longer distance you can shoot it and measure the drop accurately, the better your data will be. However, if you haven't calibrated your scopes tracking all you're doing is introducing more error into your equation.

I hadn't even considered that. ALthough for the 300 yard test I wouldnt even adjust the scope.

[Mobuck]

A "Chrony" costs about $100-150. If knowing your true velocity is important, spend the $$ rather than wasting time, ammo, and wearing out your barrel.

[emcon5]

You can certainly do that, and probably get a reasonable calculation of your velocity, but I guess I don't really see the point. Velocity as a number is not all that important, beyond the fact that you use it to calculate your trajectory. If you jump through your hoops to figure out your trajectory, you really don't need your velocity any more.

If you really want to know, what you could do, to get a quicker approximation is to get your elevation zero at a medium range, like 400 yards, then shoot groups at a couple of shorter distances without adjusting your sights, then play with the velocity numbers in JBM until you get something close to matching your real world trajectory.

For example, a .308 150 Sierra SBT zeroed for elevation at 400 yards, and you shot groups that were ~9" high at 100 yards and 11 1/4" high at 200, you could work backwards to figure out your velocity was about 2800.

To get the most accurate, you need all the other variables too, sight height, temp, baro pressure, altitude.

[kilotanker22]

Yeah, I think that While I am saving up for A radar setup. I will just have to break down and buy a prochrono or something for the time being.

Would certainly help with load work up for this rifle. I have plenty of factory ammo, as well as a bunch of brass and bullets now.

[5whiskey]

OP I've done it many times, with success, before I had access to a chrono. It can work quite well. Actually I wasn't even really interested in muzzle velocity, other than it being with a good window for the caliber, I just wanted a dope for the rifle/load combo.

The longer distance you can shoot it and measure the drop accurately, the better your data will be.

Taylor is spot on here. I always used the drop from 100-500 yards. Quite effective.

My guess is that the G1 BC is not constant over the velocity range that you will be shooting

And that's another rub. Many published BCs are off. And then there is the G1 vs G7 BC. I measured muzzle velocity based off of bullet drop and advertised BC before I knew about such matters. Still always within 50fps between my math versus what a chrono told me when I got one.