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Military Guns and Ammunition

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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons, particularly in larger calibres (12.7+mm).

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Flat vs Boattail   General Military Discussion

Started 11-Oct by JesseH1234; 447 views.
In reply toRe: msg 3
JesseH1234

From: JesseH1234

12-Oct

Well I know some match rifle bullets are flat base specifically because you can get a better gas seal and more consistent launch; they let less gas slip by.  I totally believe the base shape can have some drag effect even in supersonic flight, but I am just trying to wrap my head around how much.  I've dug around a lot looking for real chrono data and come up dry. 

Followup question: Say you had a 6mm fmj bullet, same nose shape, same overall length, same velocity; one was a 90 grain boattail, the other a 100 grain flat base.  Which would retain energy better during supersonic flight? 

In other words, to what extent would the added sectional density of the bullet counteract the increase in drag of the flat base?

I got to this question by just doing a though experiment re how much you could stretch the performance of the 6x45mm wildcat cartridge, where OAL is the real limitation if you want it to work in an AR platform.  Flat base bullets are shorter for the same weight, and most load data I could find is with boattail bullets, hence the question. 

QuintusO

From: QuintusO

12-Oct

JesseH1234 said:

Followup question: Say you had a 6mm fmj bullet, same nose shape, same overall length, same velocity; one was a 90 grain boattail, the other a 100 grain flat base.  Which would retain energy better during supersonic flight? 

Depends on the exact shape. The 90gr bullet would need a 10% better i7 than the 100gr bullet to equal it. That is possible, as I mentioned, optimizing boattail shape has earned me the bulk of a 15% improvement in drag, and that was just by optimizing boattail shape, not against flat based projectiles.

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