Pistol calibers in pistols are handy.
Pistol calibers in carbines give you an unhandy pistol.
Here's a quote from Michael J. Durant from the book IN THE COMPANY OF HEROES;
"I aimed the CAR-15 and pulled the trigger, and the burst of automatic fire took me by surprise. Randy had put the weapon on burst mode and I hadn't checked it, but it sure was effective. One moment the Somali's head was there, and the next moment he was gone. I can't say for certain but I think I killed that man.
I looked at the weapon in my hands and thought, This is what we need, not that MP-5 piece of crap. The heavier bangs of 5.56 mm ammo seemed much more effective than the 9mm "pops" from my MP-5."
end quote.
I don't know but given the same size, I'll take a bigger cartridge.
Carry on.
