As far as cartridge dimensions go, that should work fine. There is one thing that must be taken into consideration though. Make sure that the 7 RM has a 26" barrel, and that the 7 STW cartridges will fit in the Magazine. I am not sure of what manufacturers make a 7 RM with a 26" barrel, but the 7 STW would need it to take full advantage of what the cartidge is capable of. If I couldn't find a 7 RM with a 26" barrel, then I would either just keep it 7 RM or buy the 7 STW. I am no expert, so I don't know if you would run into other problems with internal pressure, etc, so I would definately as a couple of competant gunsmiths before having this kind of mod done to a rifle.