Thats the 300 terminator, which is a lapua improved with a +p throat. That's a beast of a cartridge. Barrel won't last long! You're best bet is to have your action opened up for a wyatt's cfe9 4" mag box, and bolt stop modified to let the bolt come back further to pick up the case. Not all actions are capable of this, so check into that before going down that road. Even 338 cip length 3.85 aics mags only hold cartridges of 3.75" long, so going to an M5 bottom metal doesn't get you much over your current setup.