Per Ric's request - My "Why":
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Mossberg 500 20 gauge
Ruger SP101, 3", 357 Mag
TC Contender Carbine, 357 Mag
TC Contender Carbine, 223
TC Contender Carbine, 30-30
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I live in Northwest Ohio, where it's very flat. The only "big" game is white tail deer and we can only use MLs, shotgun, certain handguns and rifles for those certain handgun cartridges. Small game - rabbits and squirrels, dove. Pheasants if you "buy" them. We have plenty of varmints, pests and predators that like to destroy property and eat small livestock, cats, small dogs..... Coyotes are very common, but there are a lot more 'coons, skunks and 'possums looking for an easy meal and they are much less apprehensive about approaching the place.
I've worked on this concept for years and believe I have avoided the old bad habit of rationalizing what I want into what I need. I see some brutally pragmatic choices in others' lists, which is impressive for what short time it took to make the decision. Took me a long, long time.
Mossberg 500, 20 gauge: I've seen these guns get some of the most horrendous treatment and still work. Everybody needs a shotgun even though I rarely use one myself. Home defense is why I choose the pump over the H&R single shot that fills that slot currently. 20 gauge because that's what I've always used, I have bad discs in my neck and they (20 gauge shells) usually sell out a little slower than 12s when there's a panic. I don't reload shotshells. I buy the cheapest birdshot on sale just before/after hunting season.
The 357 Mag in the TC Contender Carbine/Ruger SP101 Ruger SP101 357 Mag, 3":
I like my 44 Specials best, but the 357 makes more sense for me as an "optimum combination of compromises" in my situation.
"Big enough" for deer if I need it and legal in my State;
"Small enough" for small game common in my area;
"Just right" for most pests/predators at common ranges at which encountered;
Can be made very quiet w/o a can and still be effective on pests/predators;
Common, easier to find brass, which is also easier to prep and maintain w/o a lot of tools;
Can (DO) use small rifle primers, which I have to have for the obligatory 223 anyway;
I have some brass that takes large pistol primers in case things get really, really tight on primers;
Very amenable to easy-to-cast bullets, the largest of which (200 grains) uses less lead than my smallest 44, and the smallest of which (125 grains) is a real lead miser;
Loaded ammo is more compact than my big bore ammo;
Works with almost any pistol powder;
Cheap and to reload;
My rifle is under 32" long and under 5#, my revolver fits in a pocket but is ultra-durable and handles any reasonable load level without hurting me or the gun;
Commonality between a rifle and revolver;
Can use 38s if I have to;
Amazingly fun to mess around with, which comprises 99.8% of my actual firearms usage;
But then, you guys already know all this. I ignored (despised) the 357 for decades but could neither deny nor escape it's versatility and gave up. If not for my love of the Ruger "Six" Series of DA revolvers which forced me to use the cartridge, I may have missed it entirely.
TC Contender Carbine in 223: Because, everyone should own a 223 chambered in something. I like the 222 more but if I have to narrow it down, I'm going for a few extra fps, cheaper and easier to find brass, even store-bought ammo if I have to. This is the only thing I still shoot jacketed in and it sits around a lot waiting for a coyote that has studied the ballistic tables on the 357 and manages to stay just beyond my ranging comfort.
TC Contender in 30-30 because, it's very much like the 357 Mag. in many ways I mentioned. A Savage 24V 30-30/20 gauge was my first center-fire rifle and I learned a lot about its capability as a varmint round as a kid. Its other uses and attributes are common knowledge. This one is the most superfluous of my choices since it's not allowed for deer in my state, but it's ubiquitous in ammo and component availability and offers a contingency in case I fall short on some component in the 357. Can't have all one's eggs in one basket. This one is just a barrel right now, so I'd be forced to get another action and I'd think about going the G2 route this time except that two or three Contender actions is like having one with two whole parts kits. I may be pushing the intent of the original question by forcing my worst-case mentality into my justifications.
This works for ME in MY situation. Honestly, if I were to make a suggestion to anyone else, I'd most likely use the core of all the other lists shown here. Sorry that was so long.