Can it shoot a .661BC at 2860fps?
If you run a faster twist barrel it can. There are some .277 cal bullets with that high of BC. You might run into some COAL issues depending on the magazine in your rifle.
Seriously I don't know why you want a sub 7.5 lbs magnum rifle. I think a 7-08 would do 90% of what the RM would do without all the recoil. I'm not recoil shy and realize that this isn't a shoot often rig at the range but a hard hunting rifle.
I have a M70 EW .270 in a M70 FWT Edge stock with a VX3 3.5-10X40 CDS mounted in Talley LWT it weighs in at 7 lbs 13 oz. It's fun to carry and pleasant to shoot as long as I'm not pushing 150's too hard. 2900+ with Mag Pro is reachable but 2790 shoots pretty nice and isn't as hard on the shoulder, but I stick primarily to 130's.
