All of the OLY SUM barrels I used to have shot under 1/2" at 100 yards with whatever ammo that particular barrel liked.
I bought a new one almost two years ago, and it would only shoot 1 1/4" at best.
I sent back the complete upper to OLY for their opinion, and the gunsmith said I had a bad barrel, and they did have some problems with their 1-8" twist barrels, in SOME production runs.
Since then, they have supposedly replaced the barrel 3 more times, and they even sent me a test target that was SUPPOSEDLY shot with that upper, but, I still couldn't get it to shoot less than 1 1/4", so, I took it to a local barrelmaker to have it borescoped and one of the lands were "scalloped" all the way down the barrel. The owner of the barrel shop said there was no way that barrel was going to shoot accuratly, so I it back to OLY for the 5th barrel. I told them as much problems as I've had with their 1-8" twist barrels, to put a 1-10" twist on it, and MAYBE I'll get a good one.
Only time will tell.
I used to swear by OLY SUM barrels,but, at this point, I don't think I would reccomend them anymore.