870s do not breed, but they do multiply, Horny Toad. A couple things...
A rifled barrel is not essential for deer hunting in many places. Smoothbores work fine up to 50 yards or more with slugs, and practice. Some places still require buckshot, check with your local Critter Cops.
FYI, my loose record keeping shows average shot distance for the last decade runs maybe 32 yards, all weapons.
A short barreled, "Serious" shotgun, especially an 870, has lots of application for fun games, including hunting and clays. One of a shotgun's assets is versatility.
Add a longer barrel, and yes, the MM will take standard 870 barrels, you have a better waterfowler and clays tool. Or like folks have said, just get another 870. Practice with one is practice with the other, and fast doubles on clays with get you operating your "Serious" shotgun better.
HTH....