Since you already have a 54 cal. rifle, I would go with a 16ga. SxS. Though I have many SxS shotguns,my usual perc. gun is a 14ga. SxS that I use for anything with wings. My other gun is an 18ga. SxS flint that gets the same use. Just depends if I want to use perc. or flint that day. I am so used to shooting a SxS, I see absolutely NO down side to a SxS, I see only the upside of that second shot on doves, turkeys or what ever. Getting doubles on doves is still very rewarding to me! You can see my SxS 14ga. in the hunting section about turkey hunting. If your components properly fit, the 2nd barrel load will not shift. Never, ever assume that a gun marked 20 ga. or any ga. tells you that it takes a certain size of components. All guns seem to be a little different in sizing and your gun marked 12ga. may actually be better with 13 or even 14ga. components.