Figuring Muzzle velocity

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by daveyj, Feb 5, 2009.

1. daveyjMember

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I am still having some issues with my range data matching my exbal data. I think that I am needing a more accurate muzzle velocity for initial entry. My question is, how do I get this?

I have measured temps and hold off until I get 58 degs+-

I take a number of cold barrel shots and average, and do this over several days at the same location, do I need a correction for actual muzzle velocity over the 12' from the actual muzzle to my barrel, or does exbal factor this in?

2. ilscungilliWell-Known Member

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You may want to check out the jbm site (JBM - Calculations - Trajectory. The calculations on this page will give you the actual muzzle velocity based on your readings at 12 ft.

3. J E CustomWell-Known Member

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Most of the time the problem with the two forms of data not being in agreement is the
listed BC by the bullet maker and the actual BC of the bullet.

Try to determine the actual BC buy shooting at as many different ranges as possible
then use that number and it might solve the differance.

J E CUSTOM

4. daveyjMember

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So Ive worked out the muzzle velocity issue, turns out to be around 6fps is all. but the bigger issue is still BC and I dont see how shooting at differant ranges will give me a BC number to work with. I know the BC listed by sierra for the 180/190 is not accurate and I have no means of entering my exact fps to get a correct BC at muzzle velocity? Any ideas?

5. britzWell-Known Member

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There are a few ways to determine the BC of your bullet. One is to shoot through a Chrono at Two different distances and use a calculator like JBM - Calculations - Ballistic Coefficient (Velocity) and then you will know. However, I never liked the idea of shooting over a chrono 300 yards away (the longer the distance between the two readings the more accurate your BC will be.

Another way is to simply sight in dead on at 100 yards and measure the actual drop your impacts are a several different ranges. Lets say you measure at 100, 300, and 500 yards. You will be aiming several feet in the air by the time you get to 500 yards. Last summer I tested my rifle in this manner and simply took a 3' x 3' piece of cardboard and just had an "aiming point" raised above the target. I'd measure the distance between the aim point and the impacts. Then you go back to your balistics calculator and play with the bc (only the bc) until you find readings that match your actual results. Then you can use your balistics calculator and test your results at farther distances.

The key to doing testing is to have a "control" and to only adjust one thing at a time - trying to focus on one variable. So, never adjust your scope when trying to find the initial BC because your scope adjustments may not be exactly what they are advertised as.

Good luck, Mark.

6. MikecrWell-Known Member

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The Sierra BCs you 'lookup' in Exbal are accurate, but only within listed velocity boundaries, and under Std Metro conditions.
If you select a bullet with your lookup, Exbal will drop the highest listed into BC entry. But you can(& should) enter your BC and muzzle velocity and local shooting conditions.
As described, this is whatever it takes to match your results.

7. MomanWell-Known Member

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daveyj, have you tried a trajectory validation for this?

8. daveyjMember

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so perhaps I am going about this wrong. I am trying to refine my entries to dial in my POI and you folks are suggesting that I use my POI to refine my BC?

9. britzWell-Known Member

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possibly. Published BC from some bullets are not exactly right. Your scope may not be moving .25" per hundred yards per click either.

Make sure you are entering correct elevation and atmospheric conditions too.

10. lever-hedWell-Known Member

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they wont match. actual data supercedes calculated data.

11. MomanWell-Known Member

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Davey, see if this helps. Enter the data as accurately as possible into Exbal. Be as accurate as you can with this including all of the atmospheric data. Then, shoot a 3 shot group at a known distance, for me this was 675 yds. It's best if you shoot at a target or marked plate so you can identify and measure your POI. At this point, you want to measure your POI to see how low or high you were at this known range. Take the 3 shot average. Now, go back into Exbal and click Options, select Trajectory Validation. A box will open up with several small boxes inside it. You will be working off of the top row of three boxes. In the left box, enter the exact range you were shooting at, get this info from your laser rangefinder. In the box to the far right, enter either a + or - (for high or low) along with the exact measurement of how many inches low or high the average shot is for your group. For a 4 " high shot, this would look like this; +4. At this point, select calculate, then select apply. This will validate your drop sheet for this round. You will notice that it will automatically update all of your drop figures throughout the chart.

12. daveyjMember

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I cant believe I havent done this before. I will give it a go and get back to you.

13. jwp475Well-Known Member

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Yes it will correct the data, by adjusting the velocity. When I first chrongraphed my 338 Lapua with the 300 SMK I got a velocity of 2820 FPS. I found that I was quite low at 973 yards. The trajectory corrected the velocity and my drops were correct. But I didn't want to go that route at first so I adjusted the BC to .830 and that did not correct enough. I then decided to check my zero and I re-chronographed my load and this time I got a velocity of 2791 FPS average. Useing 2791 FPS my drops were and stil are right on. The trajectory validation had predicted 2790 FPS. I'll go that route from now on. Proper inof imput means proper info out put IMHO