I have an 11.5" upper to go with the various 20" ones I have kicking about.
Accuracy. 4" at 200yards with an Elcan popped on, rapid fire, 10 rounds.
Noise. Yes, noisy, but wearing muffs, not bad. In a confined area for "Social occasions" I would imagine that it'd be rather *LOUD*
Muzzle flash. Depends on the ammo, but it is quite pronounced. Distracting at night/evening shoots.
Reliability. Flipping around "The Black Rifle II" and found that the US military was seeing bolt locking lug failures starting to occur at the 5000 round mark with short-barreled rifles. Mostly (apparently) due to the increased gas pressures encountered due to the short barrels. Expect to be harder on your brass, and your bolt. Mind you, most folks won't fire 5000 rounds through their AR anyhow. So far, no problems with mine, but I'm keeping a good eye on things.
NS