A pistol grip doesn't belong on a mossberg 500 and no it will not reduce recoil. If you "brace for it" you won't be able to hit the broad side of the barn with it. Get a 20 gauge mossberg and add a nice soft recoil pad and you will be good to go.
The hardest hitting basterd I've shot is my Mossberg 500 with 1 ounce slugs, makes my 6.5lb 30-06 feel like a nerf gun. Don't put your shoulder at risk for further injury. AR's don't even really have any recoil to speak of, you mentioned that in your post... a 12 gauge is way more violent. 20 gauge is nice though, and if you just want a home defense type setup that would work well.
Shotgun loads vary bigtime in felt recoil though... bird shot and game loads won't kill you in a 12g but is that what you want at your bedside? maybe you do maybe you don't... just warning you that the heavy slugs will beat your wounded shoulder up.
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