Heavy barrel.
Read up on pros and cons of a heavy barrel.
As I understand it, they "retain zero" better than a lighter/thinner barrel under rapid fire.
That's the theory anyway and I don't know what constitutes "rapid fire".
I have one 19" carbine with a HBAR (heavy barrel) profile - my first AR15. I didn't know anything about barrel profiles at the time and always assumed that all AR15s weighed the same.
Holy hell - when I built another AR using a standard M4 profile I realized just
how much heavier my first carbine was.
That original gun is
not one that I'd choose to carry around if I had to.
http://1.bp.blogspot.com/-uHvoZM9XdW...b-profiles.PNG
Clearly a PISTOL barrel is going to weigh less, but there will still be a difference.
My pistol pictured earlier uses a heavy barrel and the pistol is heavier than you would think by looking at it.
Maybe that has a positive effect though - perhaps it helps keep the muzzle from climbing?
I'm not a barrel expert, or an AR15 expert by any means. Just sharing some things I've discovered over the course of seven builds.