Some simple math about effectiveness of SBRs

bill_bull

Newbie
I wanted to build an AR pistol and was researching what caliber to build it in. Everyone says 300 BLK is great for an short barrel configuration, but I wanted to know why. So I did some super simple math, and thought it was pretty good at showing why 300 BLK was more effective than 556 in short barrels. Please understand that I know so many more factors go into making a short barrel weapon effective like twist rates, gas system lengths, bullet weights, and so much more, I just think this math is interesting.

The basic point of increased barrel length is to give the expanding gases from the burn powder more time to push on the projectile and make it go faster. So I looked at the force being applied to the projectile at peak pressure for 556 vs 300 BLK.

_________________________________556_______300blk
Bore Diameter (in)______________0.223_____0.308
Bore Area (sq in):_______________0.039_____0.075
Max Pressure (psi):_____________62,000____55,000
Max Force on Projectile (lbs):__2,422______4,098

So the 300blk barrel has 1.9 times the cross sectional area of the .223 barrel, so even with less peak pressure the 300blk projectile has 1.7 times the force on it. Of course, pressure changes as the bullet moves down the barrel, but this shows the efficiency of a larger caliber platform with a short barrel. It's simply a function of increased cross sectional area of the barrel.

So I built a 10.5 300 BLK with an integrated folding lower receiver, and it's neat.
 
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