guns

There have been product for shotguns on the market that go into that deep screw counterbore. I'm nor sure which ones are still on the market. I don't know that any of them were any more effective than just adding weight to the gun.

If you managed to bubba an AR buffer in there, it probably wouldn't do anything but make the gun heavier. The AR buffer is acted on directly by the AR bolt carrier...no carrier, the buffer probably moves very little if at all under the recoil impulse alone. And if it did move, I would be worried about it eventually cracking the stock.

There's likely purpose-built products you could try.

Also, changing the buffer weight can impact the reliability of your AR. Don't just put a couple mags through and call it good. Ammo, fouling, even ambient temperature can trip things up. It's common for competition shooters to fiddle with that stuff to get the maximum shoot-ability they can, but there's a reason the mil-spec buffer is the weight it is.