I've fired one, for three rounds of a relatively low-recoil small shot load. Meh, but it was something I could shoot one handed with my arm fully extended and not really be bothered by it, but I don't mind some recoil that much. (I have a lot of mass, so the force needed to accelerate it is pretty impressive, and I lean into my shots.)

Personally, if you are worried about recoil, I'd look at something other than a shotgun. A pistol caliber carbine has lower recoil, more ammunition, and is more accurate at longer range. A good lever action in .357 or .44 will cost about as much, and isn't much harder to work in terms of action (is harder to reload, but you have twice as many rounds), while still giving you a nice, intimidating metal-on-metal cycling noise.

The Mossberg is a great choice. Just make sure you have done everything else you can--motion lights, trim back tall bushes and trees next to house, replace with cactus (if you're in the right climate) add the 3M film to help shatter-proof your windows, etc. As you obviously know, the Mossberg gets used when every other level of defense has been defeated. As I've had the opportunity to learn (unfortunately), there's nothing like the feel of a good gun butt in your hand when something goes bump in the night.

The knoxx stock does work. I owned 2 of them. 1 is still installed on a shotgun, the other has been retired.

While they do reduce the amount of recoil FELT in a shot, they do it by a series of springs which actually increase the recoil pulse, if you are use to shooting say your mossberg fast, switching over to this stock will more than likely slow you down since your timing will be off. This is what happened on my 870P. I went back to a plain jane speedfeed and could not be happier.

On your example I will also say stay away from any pistol grip stocks. A mossberg has a thumb safety on it, if you go a pistol grip style stock, you need to move a hand off the firearm to disengage it, OR just leave it off. Either in my book are bad choices.

Other things about knoxx stocks. They weight more. the grip protrudes from the firearm more and can get caught up on things in the tight areas of a house. There are alot of parts, IE more possibilites to break something. And yes the knoxx has broke, there were a few photos on shotgunworld forums about it.

I feel better with a speedfeed stock and forend (or even better the surefire equipped light forend). I have 5 870s, a Benelli M4 and a a M590A1. With the exception of 1 870, they all sport standard sport style stocks, even the benelli. Best thing I did to it. But this is a opnion related matter. If you like the PG style stocks, then go for it, I however did not.

Uploaded a before and after for you.
After- how I like them


Before

Mossberg makes a good shotgun. It is hard mess up such a simple weapon design. I would recommend using bird shot loads instead of buck shot for inside the house. If you have family in the house with you, that is. The bird shot energy will be dissipated in the walls and less likely to penetrate, causing injuries to the ones you are trying to protect. A direct hit on a grown man with bird shot at the ranges you are dealing with inside a house will knock him down hard. If you are living alone, then buck shot is just fine. I like to mix buck shot and slugs, just in case you need even more penetration.

+1 to Speedfeed stock and SureFire forend.

That's the elegant solution for a HD shotgun.

My first consideration for reducing the recoil of a shotgun is to get it aggressively ported. I have a couple of barrels that are well ported, and it makes more difference to me in recoil than any buttstock add-on I've tried (the Knoxx is a system I haven't experienced yet, but utspoolup's observation rings true). You can get a shotgun barrel ported for cheap, probably less than the knoxx stock you are considering. Not only will it help with recoil, it will also reduce muzzle climb. I have a Mossberg Ultimag with factory ported hunting barrel and it reduces the recoil on a 12 gauge 3 1/2 magnum to about what I feel shooting 2 3/4" skeet and trap loads, in other words, pillow soft.

The beauty of porting a Mossberg 500 barrel is that you can get interchangable barrels for it, without having to alter the rest of the gun.

Find a local gunsmith with a good reputation and they should be able to port it for you no problem.

I picked up a simple rubber one from a sporting goods store for something like $30.

What do you think about target load? I have a large box of target load sitting beside the box of clays and the thrower, I figure grabbing a sporting clay gun rather than a home defense gun looke more innocent.

I wouldn't be concerned about appearances. If you have to draw down on someone, a 12 gauge aperture is going to be just as intimidating whether it looks combat ready or more aesthetic. (Memories of John Wayne pulling up a Greener side by side in the face of a man about to attack him with an axe come to mind)

Target loads for in home defense are more than adequate. I much prefer them inside the home, for the reasons mentioned previous. Shooting an invader at close range with a target load of #7 or #8 shot in a 2 3/4 inch round, even at modest shotgun velocities, is still going to hit the opponent with a lot more energy than even my hottest 44 magnum loads out of my Super Redhawk. Shot dispersion, even out of an 18" cylinder bore barrel is going to be minimal. I would expect the pattern to be no more than 12" across shooting the length of my bedroom, which is a good 25 feet (based on a typical 60" dispersal at 40 yards). Lighter loads with smaller shot size generally pattern tighter, and the overpenetration problems are greatly mitigated, and the reduction in perceived recoil is significant, and you are still taking shots at someone with a big bore shotgun, which they will be fully aware of even if you don't hit them the first time.

I always tell my wife and daughters to keep the butt against your shoulder, aim for the belt buckle, and remember how many shots you've taken. The last thing you wannt to hear in a gunfight is the click on an empty chamber, for it also tells the opponent you are now likely unarmed (except for wielding the gun as a club). In home defense, I don't concern myself with reloading the gun. If 8 rounds don't get the job done, then more likely won't be any better, and you probably are just wasting your ammo anyways. There's no reason to fire for effect with a shotgun in a house. After the first two rounds, everyone nearby will know what's going on.

+1.

If you're dead set on the 12 gauge then I would try a nice recoil pad first. Like others have already stated; your ammunition choices are going to play a huge role in the severity of the recoil.

If you're not dead set on the 12 gauge & the recoil is a major drawback then you might want to consider other options. A carbine, such as suggested by ironraven, is a real pussycat in the recoil department.

ETA...
I hate to say it but I witnessed some failures on Mossberg shotguns years ago that left me a little jaded. I would personally prefer a Remington or better.

I have the Mossberg 500 and the Remington 870, similarly configured, and I prefer the Mossberg. But I like the Remington too. They are both fine, I think it's a bit like what ice cream you prefer, vanilla or chocolate.

As far as the Knox recoil pad...I don't really think that they are needed, but some folks swear by them. I guess it all depends on what you like. I don't mind the buck of a 12ga, my wife didn't like it, so I got her the Mossberg 500 in 20 Ga - at 15 feet, 12 or 20 ga is going to put a hole in anyone, so it's not a matter of one or the other being a better "stopping" weapon, it's a matter of which one you can actually USE.