12 & 20 gauge shotguns shoot at about the same velocities. So do 410s for that matter. The difference is in the weight of the shot charges. The 12 ga. holds more shot, so more shot at the same velocity recoils harder than a 20. We buy 12 gauges, every shotgun has a 3 in chamber, most ammo is 2-3/4, but we always seem to end up with at least some 3 in. Our current house ammo is Winchester Ranger reduced recoil #00 buck. We don't allow 3-1/2 in 12 gauge anything in the house. This is consistent with our house policy of all 1911s are 45 ACP, and all magazines interchange, all 38 revolvers are actually .357s, but we keep both 38 Spl & .357 ammo on hand. All AR 15s have 5.56 chambers, but we buy 223 commercial ammo for the most part. We try to keep things simple.