I have a 16" Colt H-bar barreled upper mounted over a Colt lower. I put this rifle together from parts I picked up here there and yonder. The M-4 ramped barrel came from the corner of a buddies barn. It was a take off he'd replaced with a fat stainless target barrel. The M-4 marked upper came from a gun shop I frequent. We slapped a round free float tube on the upper when we mounted the barrel. I used a Colt M-16 bolt carrier assembly I picked up at a gun show.
When I took the rifle to the range to zero the scope I'd mounted, I could tell immediately it was going to be a shooter. I always use whatever FMJ ammo I have laying around to get a scope on paper, and zeroed, and it was shooting them very well. After getting the scope zeroed, I began working up loads for the rifle. It was printing under an inch right from the start. In no time at all I had it shooting .5 MOA out to 300 yards with my reloads and a couple different factory loads. I think this is excellent from a 16" barreled carbine. My Colt H-bar is a shooter, and and I'm very happy with it. Even though it's an H-bar, it's not noticeably heavier than my 6.8 SPC with a 16" recon barrel.
