The number of rounds fired in the average engagement (engagement = 1 shooter vs. 1 target) where shotguns are involved is low- one or two. This is not to say that you might not have the bad luck to be greeted by a pack of five or six would-be home invaders or the like. You might then have five or six sequential engagements in rapid succession and be forced to reload in action, or to transition to a sidearm if one is available.
That is a nightmare scenario of course. But you should be working to hone your skills for a worst case event, and with a tube magazine fed shotgun there is no more critical skill set than running the gun. Ammunition management is a key component of that skill set. That means not only keeping the gun loaded but having it loaded with the most suitable ammunition for the job at hand, even during an ongoing series of engagements.
I am not talking about skillsets for 3-gun or shotgun side matches here. The shooting games teach some very useful skills, but they also instill some very bad and potentially fatal habits if the practitioner is not careful what habits he or she is building. One of my favorites of Murphy's Rules of Combat: "Incoming fire has the right of way." Cover is critical, movement and movement to cover is essential. Standing flatfooted in the midst of a gunfight can get you killed. Not only do you need to have good skills at keeping a tube magazine shotgun fed, but you need to be practiced at doing it on the move and/or from potentially awkward positions. As if it weren't already enough of a problem, right? Well, life is hard in the fast lane, and everyone knows where to find "sympathy" in the dictionary, I'm sure.
Everyone gets to start with what's in the magazine. I suppose that's why a lot of people get all wrapped around the axle looking for the longest magazine with the highest capacity they can fit under the shortest barrel they can find. Louis Awerbuck says, 'No magazine is ever big enough.' Then he sets out to prove it to you in class, and by George he's right. When you have a drill that requires you to run 15 rounds down the barrel as fast as you can stuff 'em, no magazine is ever big enough. Your ability to load the gun, keep track of what's where, if there's a round in the chamber, if there's a round in the magazine, is critical. And if you cannot keep track of these minor details (and under the adrenalin-kicked pressure of a gunfight, few people can) then your ability to determine the status of the gun and fix what needs fixing will have to do as a substitute.
Louis' answer to these problems is reasonably simple. He advises ALWAYS running the bolt, even if the gun clicks empty, run the bolt. He advises perfecting one loading drill- loading through the loading port and not worrying too much about ejection port loading. He advises loading the gun at the shoulder with the muzzle dropped just enough to take some of the weight off the gun, keeping the firing hand in position. He insists that you NOT look at the gun but keep your focus downrange on the target area.
To get started on the process of determining the status of the gun, if a lull in hot and heavy action permits, press check the magazine tube. IF:
1) you feel an empty hole, you know your magazine spring is bound up somewhere in the magazine tube and you are in trouble deep and dire. Time to transition if you have a sidearm, or else make up your mind to go to work as effectively as you can manage with the single shot shotgun you are now holding. Get back in the fight no matter what you have to do. You quit, you die. You flap, dither and screw around, you die. Real simple...
2) you feel your follower, you know the magazine is empty. There may or may not be a round in the chamber at this point, but you KNOW the magazine is empty.
3) you feel a primer, you know there is at least one round in the magazine, perhaps more. You do not however know for sure there is a round in the chamber. You might have short stroked the previous round, being sure is always better than wondering when you can be sure.
Given 2 or 3 above, next step is to press check the chamber and make sure there's a round under the hammer. With practice all this takes very little time in reality, under pressure it only FEELS like an eternity. It's funny, the flashlight people talk about what an advantage it is to be 'hiding behind a wall of light.' Well, I got news for 'em- that's NOTHING LIKE as reassuring as hiding behind the curtain of lead that a practiced operator can dispense from a 12 gauge repeater. It's pretty discouraging to have the roaring beast in your hands go utterly silent when you are under pressure to keep dinging targets though. You will want to get it back up and running (roaring) as soon as you can.
If you discover an empty chamber with your press check, you will want to get the chamber loaded ASAP. The chamber has to be loaded for the gun to fire again, after all. That means either chambering the round you know is in the magazine, or loading a round into the magazine and chambering it.
Then you load as many rounds as you can into the magazine as quickly as you are able. And you do all this either on the move to cover or from behind cover. Sounds like fun, right? And do you want the first time you face all these challenges to be 'for real,' in the midst of your own nightmare scenario?
No?
Then practice. Take a class, or several classes. If the best you can do is get on the list at the forum's Lending Library to see Louis' training tape, do that- so you will have a better idea of WHAT you should be practicing. But performing in class, with the pressure that an instructor and a group of fellow shooters can bring, is almost certain to do you a lot more good in real life than watching any amount of training tapes. Unless you are a graduate of at least one formal shotgun class, and/or are an experienced competitor and already have whatever benefits that 'stress inoculation' from competing in front of others can bring you, then IMHO you really need to consider getting some formal training and/or starting to compete in local 3-gun or shotgun side matches. The process can do you a lot of good in developing your defensive shotgun skillset, if you do it right.
All this has been a pretty long trip around the elbow to get to the thumb, in view of the original question. But I hope it's useful (if you think it isn't, feel free to ignore it). Personally I like short shotguns, and short magazine extensions- my preference runs to 18" barrels and 2-round extensions on my 870s. I like Sidesaddles of the 6-round variety, given my druthers. Of course, since it is my wife's needs that matter more, three of the 870s deployed here have standard 4-round magazines with no extensions and 4-shot Sidesaddles. "My" 870 is handy to my side of the bed, the others are ... here and there about the house, let's say.
My pajamas don't have pockets, so if I have to grab an 870 at 0-dark-thirty, what I am going to have is the ammo in or on the gun. With 'my' 870 that's 5 rounds of Hornady 00 in the magazine and 6 Kent/Brenneke KO slugs in the Sidesaddle. We live in a very rural area, there is no one else in the house but the two of us and penetration is what the bad guys need to worry about here. I know this is a luxury some folks don't have, so ammunition selection may be a different problem for others reading this.
But this is a thread about keeping ammo in the magazine, not what ammo to have available. I came to follow the basics of what Louis Awerbuck teaches about shotgun loading on my own a good while ago, it is what I practice on my own. By that I mean loading the gun at the shoulder with the support hand from a Sidesaddle, doing all the loading through the loading port unless the gun is reduced to a single shot by a magazine malfunction, and finding the loading port by using the front of the trigger guard as a guide (one of the reasons I like 870s is that fluid sweep cast into the front of the trigger guard- it's like a runway into the magazine).
One more thing. On a day when you have some spare time for fieldstripping and reassembly, load a shell into your magazine backwards and see what happens to your gun. Remember that shotgun shells will go into a tubular magazine backward just as easily as frontwards, and 'teach' your support hand to always always feel for the rim of the shell as it is positioned for loading. At best you will only have to pull down the magazine tube assembly to straighten this one out, but it is an experience you should get under your belt before it happens in the middle of your gunfight. Think of it as a motivational exercise... and as always,
Stay Safe,
lpl/nc