I guess I need to restate some things here. The buffer in the AR rifle is designed to act as a dead blow hammer when the bolt locks up.
The mass or weight of the bolt carrier and buffer do reduce the recoil by about 30%! Verses single shot no gas action at all.
When the high pressure gas launches the bolt carrier and buffer into the buffer tube it has to push against the upper and lower receivers mass slowing their acceleration and reducing recoil.
“This happens about the same time that an effective muzzle brake kicks in”
So, by adding weight to either bolt carrier, buffer or both you would be reducing recoil.
The buffer spring absorbs the energy in the carrier group and buffer and delays the transfer to the shooter untill it returns the bolt.
If you’re going to add weight to the carrier and buffer you should use a heavier buffer spring to get full benefit and reduce wear.
Same amount of energy. Just transferred over a longer period of time.
Check out
http://www.heavybuffers.com/images/Buffer%20Table.jpg A lot of good info there.
The AR15 X-Heavy's should do the trick as far as recoil goes. but, Will effect lock up.
Keep in mind that about 70% of the energy has already been transferred to the shooter “Primary recoil” before any of this occurred.