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Suppose you're designing a new blowback carry pistol (pic related).
Assuming an optimal bullet design for all, which of these four calibres should it be chambered in?
>31gr .22 @ 2350 fps (~400 FPE, basically 5.7x28)
>95 grain 9mm @ 920 fps (~180 FPE, basically .380 ACP)
>78 gr .45 @ 1160 fps (~230 FPE)
>165 gr .45 @ 550 fps (~110 FPE)
>>
>But OP, where are you pulling those numbers from? Your ass?
The 31 gr .22 is just a standard 5.7x28 loading (SS190), as is the 95 grain 9mm (Winchester Ranger .380 ACP).
I really wanted to see what a .45 bullet could do in a blowback gun, so I started looked at the 78 gr Liverty and 165 Federal bullets - lightweight, for maximum muzzle velocity/energy.
Eventually I found that the practical bolt/slide weight on a blowback pistol is about 1.1lbs give or take (see source below), so I used that to do some maths.
Using a .95 bolt/calibre ratio (pretty reasonable for the listed pistol calibes), these bullets would wind up having a maximum velocity of 1166 and 551 fps, respectively, so I rounded that down and calculated the muzzle energy from that.
By the same formula, a standard 230gr .45 bullet could only go about 395fps (80 FPE!) out of a blowback gun, before either slide velocity becomes too high, or you need a Hi-Point-tier slide.
http://www.orions-hammer.com/blowback/
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