OK, what exactly is an ASP? Some kind of store but am unfamiliar with the term Thx
Shotgun ammo is bulky and heavy compared to .223 or even .308, its main use is deterence and hunting. Having some slugs that work well in your pump gun is wise, OO buck in some quantity too. #6 and maybe some #2 shot works best for hunting small game altho a .22 rifle would be more efficient if you can hit possibly moving game with a rifle bullet. IMO, 12 ga is the way to go, if only for the quantity and ease of availability. It is more powerful than a 20, but kicks more too. For a smaller or older person the 20 ga might be best.
A good pump like the Remmie 870 which takes several barrels- small game, riot and slug gun- is what i have settled upon as all-purpose, tho I have several shotties available. Stock up on hunting ammo- even #6 at closer ranges will devastate most flesh.. It is possible with practice to get pretty good with even the old-fashion slugs within 50 yards. Many a deer has fallen to the old, simple slug. There's no way the shottie, even with proper choke and sights, is gonna be a 100 yard weapon. The 12 ga. would be a good companion esp. if your rifle is 223/5.56.