I mounted this thing to an AR. As expected, hardly a noticeable difference. lol.
I have a bushy so it's commercial spec I believe, and the fit is super tight. Reeeally tight. To shorten the length, it's easier to press down on the tab and lightly tap on the ground. But it sure ain't rattling. The finish is decent. Actually, I don't think there is any finish on it. I think it's just OD-colored injected polymer. It's entirely polymer anyway. Except the rubber buttpad. Which is really nice and thick. Cheek weld feels natural and similar to the CAA that came with my bushy.
If I ever get around to buying the 870, I would definitely opt to slap this stock on it, as the effect remains unnoticeable on the already light recoil AR. Fired off about 100 rounds with it and I wasn't blown away. It felt normal. So - marginal to no effect.
It's a fun little gimmick. You can depress the spring by applying enough pressure on it using your shoulder, or any solid surface. Just make sure it's unloaded first
My guess is you'll get the most bang for your buck if use it with a higher recoil firearm.