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Figuring Muzzle velocity

 
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  #1  
Old 02-05-2009, 07:44 PM
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Figuring Muzzle velocity

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?
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  #2  
Old 02-05-2009, 08:51 PM
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Re: Figuring Muzzle velocity

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.
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Old 02-05-2009, 11:52 PM
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Re: Figuring Muzzle velocity

Quote:
Originally Posted by daveyj View Post
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?
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
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  #4  
Old 02-06-2009, 02:40 PM
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Re: Figuring Muzzle velocity

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?
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Old 02-06-2009, 03:20 PM
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Re: Figuring Muzzle velocity

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.
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  #6  
Old 02-06-2009, 04:39 PM
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Re: Figuring Muzzle velocity

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.
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  #7  
Old 02-06-2009, 06:29 PM
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Re: Figuring Muzzle velocity

daveyj, have you tried a trajectory validation for this?
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