A 5.56mm Nato round fired from a small, short weapon without a stock. You'd never hit anything if it was real.
Actually, check out Bushmaster's Carbon-15 Type 21:
It's basically an AR-15 upper and lower receiver assembly, with a shorter barrel, and the recoil dampening assembly (minus the buttstock). Imagine using a pre-ban 40 round magazine in one of those!
Another great example is the Bushmaster M17S Bullpup:

The barrel is 21 inches in length, but the weapon itself is only 30 inches. This weapon has the advantage of the AR-15 target rifle accuracy, in a smaller, lighter package.
But back on topic, I have an odd cosplay question. Anyone out there who's done an Alucard costume? I haven't attended, nor do I intend to go to any conventions, but I've always wanted to know how one gets their hands on a red trench coat, or the goofy looking hat?