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· Gold Bullet Member and Noted Curmudgeon
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The 5.7 MMJ has the advantage of using standard magazines and not requiring mods to gas system. BUT - performance ain't all that wonderful, though as Frank Barnes notes, it is efficient. 40 grain HP with 14 grains of 4227 makes 2850 fps; 40 grain HP with 12 grains of 2400 gets 3000 fps; 50 grain HP with 14 grains of 4198 makes 2700 fps.

So - been tried, works, but not so well as to become a popular thing, factory or wildcat. In a carbine, it throws brass all over hell's half-acre, so it tends to be expensive to shoot due to brass loss. I have thought it ought to work well as a conversion in a Ruger 22/77H, what with the rim diameter on the Carbine case being almost exactly the same as a Hornet. Might need a little rotor alteration on the magazine because the case body is fatter, but not that much. Never had enough money to play with the idea.
 

· Gold Bullet Member and Noted Curmudgeon
Joined
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101,817 Posts
The M1 Carbine is as good as the design will get. Its a carbine, not a rifle. Its purpose built
for .30 M1 carbine caliber ammo. You get nothing making this wheel into a square.
Indeed so - the whole design is so integrated for getting high reliability from the designed cartridge that it doesn't respond well to any alterations or changes. That includes the pressure at the gas port and the curve in the barrel (duration) and volume of gas into the expansion chamber for that tappet gas piston design. Which works fine - for whatever cartridge it was designed around.

Got to admit, if anyone successfully got a Carbine to take 300 Whisper or 300 Shadow, I'd be interested
I think you could do that - but not as a conversion. Need to be a new-build with modifications to use the pressure and pressure curve of the mentioned rounds. Plus - might have to scale the whole action up a bit because they are a fatter case than the carbine. There isn't a lot of room to put a bigger head and case diameter into the platform. Be very expensive, and (little as I like them) probably better on an AR platform. Maybe (at least) modernized with a gas piston instead of direct impingement.
 
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