50 yards is quite a poke with BB's in a 12 ga. I had better luck with an Ithaca 10g 32" full choke with this kind of load.
If you want to go the longer ranges, then #4 buck or the high tec shells in T shot are the answer.
I encourage you to try some Rem 3" 41 pellets of #4 buck if you want the cheaper route.
Chokes are sure different from barrel to barrel, so you are really in a boat by yourself in this issue. Point of impact vs point of aim is a whole nuther subject by itself(stock fit).
