I have one in a competition shotgun but haven't tried one in a rifle. all the reducer does is add weight which the Mercury adds a little more than lead as Mercury is more dense. Other guns to which I've wanted to add weight received as much lead #9 shot as I could get into a Baggie and still fit into the stock bolt. This what I did with other shotguns, mostly slug guns.
I haven't seen a need to do so with a 375, even the Savage is not too bad. I did add a different recoil pad which helped a lot- the original pad must have been a left over Browning pad from the 70's. I would start with a new pad, a Pachmyr would be good or a Kick-Eez if you wanted to add a little weight. I would also change my shooting position if shooting from a rest, get more up and down in your posture and not leaning down and in as one typically does from a bench. Shooting as if using a 22 will leave you bruised but if you can get upright your body will move with the recoil and not be very hard on you. I recently shot 30 rounds of 416 Rigby and though it did leave the shoulder a little sore, I didn't have a bruise. I fired standing from some sticks which allowed my whole body to move. This would be the way to shoot if possible.
Do what you thinks needs to be done but I would put the recoil reducer at the bottom of the list. There are other things that will make at least as much of a contribution for the same or less money.
