Cw. What are you shooting out of your 12? I think you also mentioned in a previous post that you own both the 12 and 20 also.
When I bought my USH, I was shooting a 870 with a fully rifles cantilever barrel and Barnes expander's into about 3" at 100yards. I was at the range with my friend and saw some guy shooting one hole groups with a H&R fully rifled shotgun. I went directly to the store and ordered one of them!! Got it a short time later and headed to the range with an array of slugs and great optimism. I loading it up with the expander's and shot some groups. I should call them patterns as I COULDNT cover them with a dinner plate!!! Closer inspection showed they actually where tumbling when they hit the target @ 100 yards!!!! Allot of screwing around with reg slugs at 25 yards to get it on the paper and somewhat zeroed. I switched to another slug. Better accuracy but it was certainly no one hole shooter!!!
I don't remember what I did next with the gun. A good cleaning, I'm sure of that. I went thru everything, base, rings, scope and more ammo. I got some of the then new Rem copper solids. Got back on the range and dropped the hammer on these new rem slugs.. WELL WELL Lookie here... I hit pay dirt as now I had slugs touching at 100 yards.... Mose forward to about five years ago. my supply of copper solids was just about done and they changed the loading. the new ones where not as good, but OK. Remington had a couple new slugs. The BUCKHAMMER and the Premium ULTRA COR-LOKT. I picked up both and headed to the range. I was elated with the first targets as the busk hammer was shooting a bit larger groups that the copper solids, but because it was punching 7/8" holes they where all touching!!! the Ultras shot even better and about two inches higher and perfectly centered. I had a perfect one two punch!! Buck hammers sited dead on @ 100 for everything with in 100 and the ultras for anything out to 150 and 175 even as this was a higher velocity round and as I already recounted, accuracy was not an issue.
As for my USH 20, yes I do have one. I have shot it a couple times and found it likes the Winchester 3" BRI sabot slugs. Not as good a shooter as the 12, but in fairness, I have only shot a couple different slugs thru it. It shoots maybe a 2" group at 100 IIRC.
CW