I have had 2 bps for turkey but both were 10 gauge. I havn't had any shooting experience with 12 gauge bps. The first I left stock and let a buddy talk me out of it. The second was a stalker model. With all other things being equal,a 10 gauge 3.5" will pattern tighter than a 12 gauge 3.5". This is because of a shorter but wider shot load/string thus less pellet deformation. Yall can argue with me but that is the truth. The stalker I set up to be the ultimate turkey gun and got what I wanted at the time-since then I switched over to bowhunting only,even for turkey.
I backbored,tapered forcing cone,ported,hand honed blank choke tube,indexed it to bore,and drilled reciever for full saddle mount(6 8x40),and red dot sight. At 60 yards-NOT 40-it would put the WHOLE pattern into 37"-40". Needless to say,It was the tightest patterning shotgun I've ever seen outside of outlaw turkey shoot guns.