I got a 20ga pump last year (made by Rock Island, their "all generation" deal that has a stack of random features, hey $200 wasn't bad) and luckily then ammo was available so I grabbed what I could, shortly after that all ammo disappeared and about the only shotgun ammo available was 28ga for whatever reason... Now things are coming back but I have enough buffer for a while... annoyingly a couple boxes I bought because it was available was #9 shot, that stuff will not puncture the hide of a gopher as it turns out, fine for clay pigeons but that wasn't the purpose of the dang thing... At this point it's gotten at least 30 gophers I think, I lost count a long time ago... I've had a handful of times where the firing pin didn't come all the way out and get the primer hard enough but I'm not feeding a freezer so I'm not too worried, still a good yard gun...
I like being on the low side of things, I used to use a 12ga for gophers but they kick a bit more than I'd like for yard use, 20ga is a good compromise... heck, you guys have probably seen my pellet gun thread, I need to get more trigger time, I missed the last gopher by a little...