I put a deadly wind Fibur barrel on mine, as well as the Bad Karma bolt and softer paddles as it kept breaking paint. Unfortunately my reg was damaged, which the nice people at Tippmann have replaced for me, so none of these upgrades worked. Haven't had the chance to try it properly yet but I'll let you know after my next walk on.
The hammerheads are a cracking barrel. I have one on my sierra and works a treat due to the nice tight rifling.
Another note: the pressure gauge and quick strip pins I've heard from other players are a bit of a god send so may be worth taking a look at them