I have a friend which purchased a high quality upper, identical to mine except mine is 16" and his is 18". They have 1:8" medium contour stainless match barrels with match chambers. Mine shoots nice tight groups and has been sub-MOA all the way out to nearly 500 yards. Bulk ammo shoots about 1-1.25 in mine. His shot match ammo into about MOA at 100 for one group, but then started vertical stringing. With M855 and M193 its shooting about 4.5-5 MOA. His “pet load” 60 gr V-Max shot about 1.5 MOA. What happens with match ammo and handloads is it shoots two rounds through one hole, the next two shots will be about an inch above through one hole, then it shoots another inch high. Leaving a nice .4 or .5 MOA horizontal, 2 MOA vertical group.
Needless to say, he was quite disappointed. The only things I could think of, aside from the shooter, that causes vertical stringing with an AR are barrel extension fit or a loose barrel nut. He pulled it apart and checked the torque, the torque was 48 lbs. I believe for the rail system that came with it, it calls for 40-50 lbs. He tried it again with the same results. Shooting side by side with his Daniel Defense mid-length, the 14.5 DD government barrel is out-shooting the stainless match gun—so I’m ruling out shooter error.
The barrel extension seems fairly tight. Could it be that it isn’t tight all the way around? Perhaps either the reciever or barrel extension are out of round?
Sounds like he’s on the very edge of an accuracy node. Perhaps tuning the load up or down .3 gr would bring the barrel harmonics into greater consistency.
What concerned me is with multiple loads it’s the same thing—factory Hornady, my load, his load. I will suggest tweaking it a bit. He’s getting pretty frustrated.
Did he put the upper together himself, or buy a complete upper? I’m wondering if his upper needs to be lapped for a consistent barrel extension fit all the way around. It sounds like when it heats up there is pressure coming from somewhere which is affecting accuracy.