I have Recoil Reducers in 3 of my shotguns and 2 of my hardest kicking rifles (.300 Wby and .375 RUM), along with 1" Limbsaver, Kick Eze, or Hogue pads. One of the reducers is mercury, the others are mechanical, Edwards or Dead Mule. Some are 6 oz, others are 8 oz. I put reducers in my competition Trap and Skeet shotguns in the mid '80s. At this point I can't remember which ones are in which rifle or shotgun, but they all helped to reduce the felt recoil of each gun.
I was a little nervous drilling 7/8" holes in the AA Fancy walnut stock of my .300 Wby and in the stock of my .375 RUM, but I was very careful, and everything turned out OK.
For Skeet competitions, I had my Browning Citori O/U 12 ga shotgun fitted with full length Briley 20 and 28 ga and .410 tubes so that I could shoot all 4 events with the same shotgun. To help keep the weight of the gun close to the same when shooting 12 ga without tubes to shooting the other gauges with tubes, I routed out a channel in the gun's forend and poured 6 oz of lead into it. This lead plug easily comes out when I use the small ga tubes.
I also had the barrels of my competition shotguns ported which reduces felt recoil a little, but more imiportantly it reduces the muzzle jump which is very important for a quick 2nd shot in doubles.
30 years ago I bought a Rem 12 ga 870 shotgun for bird hunting, but when shooting a 25 round of Skeet with it, it would bruise my cheek, so I lengthened it's pull with a 1/4" spacer and put a recoil reducer in it's stock, and the bruising stopped.
I've built the stocks on all of my rifles, and have lengthened the pull on all of my shotguns so that they all fit me.
When I first shot my .375 RUM, the recoil was so bad that I couldn't even shoot it enough to zero the scope. I then had a KDF muzzle brake installed on it and put a reducer in it's stock. I now do not have any proplems shooting it off a bench or prone.
Before I fired the first shot from my .300 Wby, I had a KDF muzzle brake installed on the barrel, and I built the AA Fancy walnut stock for it that fits me and has a recoil reducer in it.
I've used these 2 rifles on at least 11 guided hunts, and have never had a guide or PH say anything about the report of these rifles. I always use at least foam earplugs when I shoot these rifles, and I carry extra new earplugs that I offer to the guides.