My "Mossberg 500 Tactical" shotgun came from the factory with a ATI pistol grip stock with collapsible buttstock.
I wrote about pistol grip vs traditional before.
Mini-review on the ATI stock:
There was nothing particularly wrong with it, I found it comfortable and liked the ability to adjust the length. Recoil was totally comfortable with birdshot, and noticeable but not too bad with buck and slugs. It came with a shell holder on the side of the stock, which I didn't much care for. A shell holder on the side of a stock prevents a cheek weld when shooting left handed, which one might need to do for whatever reason.
The ATI stock was okay, but I prefer a traditional style stock for the Mossberg; with the pistol grip on, you won't be able to manipulate the safety without taking your hand off the firing grip. With the traditional style stock you can very easily and quickly manipulate the safety and slide release without moving from a firing grip, which is one of the strengths of the Mossberg shotguns. I chose the CompStock over a regular fixed polymer or wood stock because I intend to take a few defensive shotgun classes and shooting 250 rounds of buck and 50 slugs in 2 days is presumably tough on the shoulder.
The Knoxx CompStock for Mossberg 500 is a traditional style shotgun stock which uses a spring to reduce felt recoil. Knoxx was bought by Blackhawk, so you sometimes see it called the Blackhawk CompStock. Note: the Knoxx Spec-ops stock has a comparable recoil reducing feature but is a pistol grip stock with adjustable length of pull.
When I got the stock, my first problem was that it doesn't come with a stock bolt. The instructions mentioned something like "Mossberg owners should use their existing stock bolt". The bolt that comes with the ATI stock is way too short to work with the CompStock, so I was out of luck.
I've read that if you call Mossberg, they will send you a stock bolt for free. Another option is to go to a hardware store and buy a 5/16" hex head bolt that's 5.5 inches long.
Note: 5/16" is the width of the bolt(the long threaded part). The hex head is 1/2" and thus requires a 1/2" socket. When you look online about Mossberg stock bolts, sometimes you see people say 5/16, sometimes you see 1/2. This is why.
You'll want one that's exactly 5.5 inches long. 5 inches is too short and 6 inches is too long. I know because I bought a variety of 5/16" bolts. Stupidly, I tried installing the 6" one, and it was too long and went into the trigger group and broke something in the back of the trigger group. I was lucky that I went to Tabor's Shooter supply in San Bruno and Frank had a trigger group for the Mossberg 500 that he sold me for only $25 (he took my broken one in exchange to use for parts). This was a good deal because a new one costs about $70. I called Mossberg and they would have fixed/replaced it for free but I would have had to send my broken trigger group to them and the turn around time would have been about 3 weeks.
To install the stock bolt, you need a socket wrench, an extender(about 6 inches is sufficient), and a 1/2" hex socket. Obviously if you got some other kind of bolt besides a hex head, you might need some other kind of tool to install it, like a screwdriver or allen wrench.
I would recommend field stripping the shotgun and installing the stock bolt with the trigger group NOT installed. When you install the stock bolt, make sure that it stays in the threaded hole in the back of the receiver. If it protrudes at all into the inside of the receiver, it may interfere with the trigger or safety, and potentially cause a slam fire.
Aside from the stock bolt hassle, installation was very easy and straight forward. You will need a long thin Philips head screwdriver to install one screw. The screwdriver needs to go through a plastic hole and then go a few more inches before screwing in the screw.
The stock has a 13.5 inch length of pull, and you can separately buy a thicker pad that increases the LOP by an inch. It's comfortable to me as it is.
When I was researching this stock(and the Knoxx Spec-Ops), some reviewers said that they felt a cheek slap from the receiver moving back with recoil, and a lot of other reviewers did not mention it or said they didn't feel it.
The instructions say to make sure your firing hand is at least 2 inches away from your face to avoid hitting your face with your hand, and to put your head a bit further back on the stock to avoid cheek slap.
It also recommends keeping the thumb of your firing hand up along the top of the stock rather than wrapping around the narrow portion of the front of the stock, in order to avoid contacting your face with your thumb.
I got a chance to use it in a class where I shot about 150 buck and 40 slugs. The recoil reduction worked pretty well, I didn't really feel any recoil in my shoulder the whole time. I did get cheek slapped a few times but after a while either I got used to it or figured out what cheek weld to use to avoid it, so after a while it wasn't an issue. Update: I took another class where I shot about 200 buck and 30 slugs. This time I had no cheek slap at all. I think when people have a problem is when they incorrectly lower their face on top of the stock. You need to keep your head upright and lift the gun up to the side of your face. To tell if your mount is okay for not getting slapped, you can simply pull the stock back hard against your shoulder. The spring will compress a little bit just from the force of your pull. If you're doing it right, it shouldn't hurt at all. I do not need to do anything special with my mount for this gun that I wouldn't have done with a traditional solid stock.
Shame on Knoxx for not including a stock bolt(which cost 80 cents at Home Depot). It was a huge hassle to figure out what kind of bolt I needed to get and how to install it.
Was it worth over 100 bucks? I'm still undecided. While it is effective in reducing recoil, I'm not sure how much anyone actually needs recoil reduction equipment vs proper technique to reduce felt recoil.