Here’s an assignment for you Gun Nutz(tm):
Your task is to find a sidearm/long gun combo. Assume that money is no object, and that anything goes, even NFA stuff. Interpret “long gun” as liberally as you would like: carbine, rifle, submachine gun, whatever, as long as it’s fired from the shoulder.
Your only restrictions: the combo must use interchangeable ammunition, and it must be commercially available ammo. (By “commercial” I mean “on the shelf in a moderately well-stocked gun shop”, not “Old Western Scrounger has two boxes of it in stock for $80 each”.) The same load needs to work for both weapons without malfunctioning or blowing up the gun when used in one instead of the other.
Your combo should be chosen with an eye on flexibility. Ideally, you’ll want to be able to drop two- and four-legged vermin, so it will have to do for self-defense and hunting of small- to medium-sized game up to 150 yards or so.
Name the gun combo, and the caliber. Ready? Go!