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I have the ATI 6 position on my Mossberg, it's awesome. Adjustable, comes with butt pad and fits your shotgun too. 69.99 at BC specialty sports or whatever its called.
They're pretty cheap plastic, not very well designed.
I don't think they'd hold up long.
If you want I'll sell you the Knoxx NRS (non-recoil reducing) one I just took off my 870 (way cheaper than 120$ I might add). I found it awkward for left-hand speedloading but all the righties love them.