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Ballistic coefficient

bozo699

Gold $$ Contributor
How can one measure or figure B.C? What I mean is say you meplat trim your bullet and it had a b.c of .527 before trimming, what is it now? is there a mathematical way to figure it out? Thanks in advance.
Wayne.
 
I think you would need some more data. I think you could fire a group at 100 and 300 and measure the difference in point of impact. Then, assuming you know your velocity, play around with a ballistic calculator and figure it out. I'm sure shooting at multiple distances and further than 300 would give more accurate results.
 
In Applied Ballistics, Bryan Litz provides the results of a regression he did on the various things that determine the form factor of a target bullet, one of which is meplat diameter--by playing with the numbers for your bullet, you could come up with an estimate of the new BC.

I would suggest entering the numbers for your bullet as described in the reference section, then change the meplat diameter (and possibly nose length) to the new values, and note the effect. (if the form factor is 5% higher, then the factor will be 1.05) now *divide* the BC value you're using by that factor. Larger form factors mean more drag and *lower* BCs. That will probably get you pretty close. Or, at least, give you an idea of how much BC you're giving up by uniforming the meplats.
 
Wayne,

Yes, there is, but for any real degree of accuracy you'd still need to fire the modified bullets for time of flight. Check out Hatcher's Notebook or Townsend Whelen's book on small arms for the whole process, but meplat diameter (in calibers) is one of the variables given for the equation. They use Ingalls tables in there which are very close to the G1 model, so there's a good degree of correlation. No criticism here, as the G series of drag models hadn't been developed when these books were written.

That will get you in the ball park if you want to try some theoretical comparisons, but it's not a substitute for actual firing data.
 
Thanks guys that's kind of what I thought by what I have already read, I thought possibly someone had come up with some cowboy way of measuring that would get you close, I am not willing to burn up valuable barrel life to figure it out. I am meplat trimming my new Hybrids and noticed it took almost .010 to clean them up, then you could gain about .003-.004 back by tipping but you still loose some overall length, not really a big deal just a thought, I appreciate your thoughts and expertise :)
Wayne.
 
A lot of shooters have done the velocity measurements requied for the new BC and maybe some of them will post their results. One of the main benefits of trimming is a decrease in the vertical spread. A friend of mine uses a pointing die and it gives him about 1 min. difference at 1000yds.
 

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