All the fancy aimdots, ghost rings, side saddles, seems to hinder basic uses of the gun. Some of the pros mention it and for now, it makes sense to me.
You gain wisdom, Grasshopper. Furthermore, the fewer bells and whistles, the fewer things to get dirty, broken, or otherwise inoperable. Under combat conditions, Murphy is your co-pilot and you will end up having to make do with a lot of things you normally wouldn't even consider. Best you don't burden yourself with things you can't fix yourself on the fly.
In that respect, the Mossberg is one of the best survival shotguns going becuase of the ease with which it can be repaired (with appropriate spare parts, of course). That, and its less expensive cousin, the Maverick, the parts are almost completely interchangeable except for the safety and those parts which engage it.
12 rounds isn't enough for a combat load out with the shotgun. As has been already noted, a shotgun is not a good field weapon for combat because the weight-per-shot is somewhat dismal.
HOWEVER, I fully understand "push with the cock ya got" and I DO have shotguns in my own battery at home for perimeter defense, and you never know when I might not have the luxury of one of my carbines for an extended engagement.
Remember that if you're packing a long gun afield, weight is your enemy (but you already know that from your first statement I quoted up there). So, you're going to be CARRYING more than you will be SHOOTING, and when you're SHOOTING it will be in a life-saving capacity, so recoil is not going to be as big a deal for you. Thus, the secret is a LIGHTER shotgun so you can carry MORE ammo.
Go synthetic as far as buttstock and forearm go -- lighter without sacrificing too much strength and doesn't warp with humidity, doesn't crack or chip as easily either. If this is a combat gun and you intend to really engage real-life bad guys with it, you are going to beat the hell out of this piece of equipment and you want it to work all the time, every time, not necessarily look pretty doing it.
That means, if you have a chance, get the metal coated with some kind of protective coating...parkerizing at the very least, hard chrome or other industrial noncorrosive finish at best. Trust me. Most bad guys don't wait to attack you in good weather. With Mr. Murphy along, you'll have to engage multiple enemies you can hardly see while crouched in the mud during a torrential downpour. Make sure your firearm can handle that task.
As far as your loadout, I'd recommend being able to reload your magazine from dry at least six times. If you've got a four-shot weapon, that's 24 rounds; Five-shot weapon, 30 rounds.
If you're running a Saiga 12 with 10-rd magazines, that's about 60 rounds.
To be honest, that's STILL not enough for an extended firefight in my opinion...but it IS enough to "get you through one alive," if you're sparing with your shots and generally hit what you're aiming at.
And those of you who have shot trap and skeet know that a standard box of 25 rounds isn't light...double that if you're talking about buckshot, and you start to realize that the more shells you carry, the heavier your weight, and the slower you can go.
Putting the shells on the vest sounds like a good idea; I don't do it that way but that's just my preference. I won't dictate how you handle your ammo. The way I carried my combat ammo for my Crowd Pleaser back when I had my shotgun "for real", I carried a bandolier of 25 buckshot rounds for my Maverick 88 and later, my Remington 870. That, plus a fistful (usually another 6-12) in a pocket and I figured I was okay for an engagement.
If you want a solid rule of thumb, I'll give you this:
FOR A COMBAT SHOTGUN, YOUR MINIMUM LOADOUT SHOULD BE 25 AND YOUR MAX SHOULD BE NO MORE THAN 60 ROUNDS OF 2 3/4" BUCKSHOT ROUNDS.
Do not mix loads. Don't listen to the idiots who tell you to alternate between birdshot, buckshot, and slugs...that's a recipe for disaster and those who tell you that either haven't actually been in combat, or have read too many books, or both.
A cornerstone of marksmanship is consistency. Another cornerstone of effectiveness is being able to accurately predict the damage your firearm is going to do BEFORE you pull the trigger so you know what to look for. No one I know of, combat veterans and competition shooters alike, can actively, accurately count their shots during a string. They either tactically reload or reload from empty -- what this translates to is that if you alternate your ammunition, you don't KNOW what's coming up in the chamber, so you really have no way of using "the right tool for the right job" -- and thus, mixing your ammunition is moot. Sounds great and way smart on paper, but doesn't translate well to the real world of stopping deadly threats.
So stick with buckshot. I go with 00 buck because not only is it traditional, I know it's going to be at least 9 pellets of .33 coming out of my shotgun, which is, in my opinion, close to the equivalent of a 9-shot burst from a submachine gun at close range. This is also why the shotgun is called the "poor man's submachine gun."
It is recommended that your tactics reflect this. Engage, engage, cover, reload, engage, engage, but NEVER run your gun dry -- the advantage to a tube magazine is that you can top it off like a tactical reload every time you find a break in your shot strings, behind cover, which means you will never waste ammo and you shouldn't EVER reload from dry unless you're in the middle of an extended shot string...and if you practice, you'll understand that your shot strings need to be short, two to three, NEVER your full magazine capacity, because if you're out in the open and you run dry, you're pretty much dead.
Because as the shotgun, you're throwing out enough lead to be suppressive fire. When that suppresive fire stops...who do you think target number one is going to be?
No less than 25. No more than 60. Synthetic stock. Rifle or ghost ring sights if you can afford them, standard physical bead sight if you can't. No breakables. Keep your weapon clean and well-serviced, and for Server's sake, KNOW YOUR FUCKING MANUAL OF ARMS.
Sorry, that's a pet peeve of mine. Where a guy who says he knows fuck-all about shotguns picks up a Mossberg that's already cocked and he can't figure out how to rack it and won't listen. That's the kind of stupidity you need to avoid if you want to survive with this weapon. Know where the safety is. Know where the pump release is so you can rack a live shell out INSTEAD OF PULLING THE FUCKING TRIGGER TO FIND OUT IF IT'S EMPTY. The release is different on different models. The Winchester and Mossberg have it on the lefthand lower side of the receiver BEHIND the trigger guard. The Remington 870 has it on the righthand side of the FRONT of the triggerguard.
KNOW YOUR GUN.
And take some tactical classes. Note that they will require roughly double to five times as much as your regular loadout, or more, for the class, but remember that the class can be multiple days.
After Tactical Shotgun class, you will know how much ammo you want to load out when carrying a shotgun as your primary battle weapon, and you won't need my guidelines anymore, because you will know, instead of simply having been told.
How you carry that ammo is up to you. I like loose in a pouch or in VERY easy to access loops...Not fond of vest carry. But that's just me. I don't use speed loaders or speed strippers, but that's not to say they aren't good...it's just that I learned shotgun long before they came into vogue and my technique works for me so far and I don't see a point to changing it.
Finally, don't listen to the armchair commandos who tell you "Ah, you don't need to aim a shotgun, just point it in the general direction of your target, that's why they're so great for defense."
I'll tell you straight up that quote is pure-D bullshit. First off, at your engagement distances, with an 18-20 inch barrel (that is, from about 10-25 yards), your shot pattern from 00 buckshot (9 pellets of .33) is almost coverable by a fist. So you'd damn WELL better aim that thing or you will MISS, and that's a hell of a lot of lead to be accountable for that doesn't hit your target.
Second, your goal, when using buckshot, is to make sure ALL your pellets hit your target. A "general direction" shot may hit your target with one or two, but now you haven't fully incapacitated someone who's trying to kill you AND you've put out some high velocity lead that's going in a direction you didn't intend.
If you've ever shot skeet or trap, you will understand that you DO need to aim a shotgun, and not only that, understand a shotgun's pattern so that you can aim it in such a manner that the most number of pellets hit your intended target.
Once you do that, you can use a shotgun to its greatest devastating effect, and yes, you CAN use it as a combat weapon.
But you must understand its strengths and weaknesses, and train to play to and mitigate them, respectively, long before you have to bet your life on its deployment and use.