My tastes run to midlength gas systems in a 16" barrel.
The only thing to look for thats a gotcha in the ar15 is if your reciever has m4 cuts but the barrel extension doesnt.
If the extension has cuts you can use either an m4 cut upper or a standard a2 upper. If your upper has the m4 cuts your barrel has to have them also.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...