So I’m talking mostly about AR pistols.
I’m no ballistics expert, but here’s my understanding: You have x amount of propulsion and n amount of mass. The idea is to propel that mass down a confined tube with maximum efficiency.
More mass takes more propulsion to get to a given velocity.
Now we’re not dealing with whatever values we like. There are not only constraints to the values available to us there is also a limitation of pre-made combinations of these values.
Ideally, the propulsion you have available is completely expended at the same point your mass leaves the confined tube. For morons, the powder is burned at the moment the projectile leaves the barrel. Not before, not after.
So for off the shelf bits, this leaves you with rifle rounds designed for rifle-length barrels and pistol rounds designed for pistol-length barrels. Now, I’m not smart in these things but I’m mostly certain that your average off the shelf 5.56 ammo is designed for about an 18 inch (maybe 20) barrel. You fire this round through a 6 inch or 10 inch barrel and you’ve blown most or half of your propulsion out the barrel to no effect. You get a loud noise, shitloads of unburned powder leaving the barrel and a projectile that is moving far slower than intended.
To my estimation this is not much different than firing an off the shelf 9mm round out of a 20 inch barrel. The propulsion has expended itself before your projectile reaches the end of the barrel resulting in a slower than optimal projectile.
I could be entirely wrong about all of this and I’m quite certain someone will tell me I’m completely full of shit. If you want a short barrel, you should be using a pistol ammunition.
I totally understand the idea of a short rifle. It gives you a forearm for stability but is short enough to maneuver in tight spaces.
For me, that means pistol caliber carbine. Like God intended. Not this silly AR pistol shit.