If's just not right for you to have posted a pic of such a rig. From the looks of it, all bullet energy will be lost due to pressure reduction caused by those 10 big holes in that weird lookin' brake. The barrel looks long enough to require a goose neck trailer to haul it. Oh, and the paint job. If you ever took more than 10 steps away from it in the woods you'd need a metal detector to find it! Look at those flutes. In a rain storm they most probably act like rain gutters. Sheese! I suppose you're gonna tell me that Kirby built that thing!
Now that you're all pissed off
I'll tell you that all that is remaining for Kirby to do on mine is machine the brake , same model as yours, and put the pieces together.
I'm really looking forward to the honeymoon with that sucker
While the 300 RUM is tough to beat, rifling twist and bullet weights get to be a consideration. Also when souped up a bit it's barrel burning tendencies increase considerably.
I've been shooting a 338 RUM w/300 Hybrids and SMKs out two 1200 yards. Its a simple straight forward REM 700 LH (not trued) with a single shot follower glued in. And a tupperware stock. The only thing out of the ordinary is the Holland recoil lug and QD Brake. The only think Kirby has had to do with it was sell me the new 338 RUM sporter weight barrel for 40 bucks.
Velocity is a bit short of an Edge (100-150 FPS slower) but better than 2700. If
it didn't shoot so very very well with such great consistency I'd consider an 28" heavy fluted barrel.
I flat lucked out on it being a shooter. If you want to guarantee a shooter then get Kirby to work it from the get go.
I have a 270 AM from Kirby on a 721 Rem action which still exceeds his 1/2 moa accuracy goal for many more shots than it should have.
The 375 AM from the ground up custom is quite a chunk of change and with the limited selection of reasonably priced bullets (350 SMK) it won't get you any further than a 338 AM. The 338 AX is something to think about also. The cartridge that would make the most sense, all things considered, would be the Edge.
The Edge because it is a wildcat, which forces one to have to go to a gunsmith to put it together. The gunsmith is THE key. Most if not all of the smiths who hang around here could make a turd shot with scary accuracy.
I'm sure that which ever direction you decide to go will be a good one.