Hey Jayta98, was wondering if you ever got a chance to test this out. I have the M4 slip on pad on my 500 w/ ATI Scorpion Stock, but I honestly think it is worse at mitigating recoil than the ATI pad. Took the 500 12 and 20 gauge out to the range 2 days ago. Fired 2 rounds of Winchester Target & Game (2 3/4", 1 1/8 oz. #8 lead shot) through the 12 and 5-6 rounds of Estate target load (20g has a slip-on LimbSaver on the original stock) and my shoulder feels like I let the biggest guy I could find punch me in the shoulder.
I don't think the M4 pad has enough material to eat up the recoil mixed with such a large, flat surface area of an M4 type shotgun stock. Next time I get the chance I'm going to take both the ATI and LimbSaver pads out and see how they feel side by side, but I'm leaning toward the ATI pad seeing as it was designed for shotgun recoil though.
On a side note, the 20 gauge wood stock with a slip on LimbSaver feels like getting hit with a pillow by a little girl. I don't think the LimbSaver is junk, just not right for this application. I had a slip on one on my wood stock for my 12 gauge and loved it, but gave it away after switching stocks. There was a group of young girls (maybe 13-15) at the range with a youth group shooting pistols for the first time. When we pulled out the shotguns a few of them where eying them, so we asked their youth pastor if they'd be interested in learning to fire a shotgun. I ended up teaching them how to handle a shotgun and let them fire off a few rounds. All but one girl tried and all of them loved it. I remember shooting my first shotgun in youth group, so I was glad I had the chance to continue that tradition for a few kids.