Azshooter, Remington has made dedicated RSAUM magazine followers, but their present Q.C. problems precludes me from buying one of these as a donor left hand rifle. My Mauser test beds seem to do O.K. using the old military followers, and some rail work, along with the magnum bolt faces. I can later do any standard length belted or Ruger Magnums by using new commercial barrels. Right now I'm using old, freeby, military stepped barrels, with a couple of barrel threads trimmed off. With the elongated, by case tapering, 300 RCM at ( 53.94mm) C.O.L. I've cleaned up the old chambers' shoulders, leaving their necks and throats alone.
What do you think is possible with my 72 gr. capacity 7.92 version? With my "Schuleresque" case taper, I'm really close to your shorter 338 RCM's capacity. This old barrel is 21.5 inches long, with a shop made muzzle brake, screwed on, for about 24 inches total length.
On your long action with that 27 inch barrel, you might re-chamber the RCM to a full length 338 x 375 Ruger wildcat. This will nip at the heels of my 26 inch barreled 338 R.U.M. I have a couple of these Ruger/Schuler knockoffs, in a 8mm and in a .416 caliber. FWIW, even with my exaggerated case body taper, I have exactly the same case capacity as the 1/10th of an inch shorter, 2.5 inch long 416 Newton Wildcat.
With these Ruger parented designs, you can pick up quite a bit of capacity, just by building them in full length 375 H&H pattern actions. Personally, I'm happy to stick with my 66mm long, 8mm Ruger/Schuler knockoff, using 200 gr. Speer Hot Cores.
The real McCoy, which is the 8mm x 68mm S Schuler, is still really popular in the Horn of Africa, but Hillary's goons at State, have so far, choked off our Euro supplied ammunition for it. So, I simply "Americanized" it, to kick some sand into their faces.
My 416 is reamed with this same 8mm PT&G finish reamer, using an oversized pilot bushing. Then my G.S. did my 2 R chamber using his own set of neck throat reamers. With my case taper, my .416 wildcat is the largest bore I will try, with this finish reamer, and its companion PT&G GO gauge.
You can do about the same thing. Your stock 300/338 RCM's can be necked up to .429 to make short fat, and rimless, 444 Marlins on steroids. My short RCM parented wildcats have 26 degree shoulders and are .495" at their shoulders. Your cases have 30 degree shoulders and are a little over a half inch in diameter, at theirs.
So the Hornady 265 gr. Flex Tip bullets can be juiced up to the top of their working range, in a bolt action rifle. At least the dummies I made up with my unique case body taper, fed fine out of my M-98's box magazine.
My earlier 44 x 356BB wildcat is in a Post 64, T.E. Win. M-94 carbine, and also clearly fulfills the Seahawks' motto of "The Legion of Boom", with these 'gummie tips'. The difference here is between the capacities of the 300 Savage, vs. the 30 Gibbs, with both ratcheted up to 44 caliber.
My Winchester carbine has a re-cut 30-30 bbl., with a 1 in 20 twist, and it stabilizes both these Gummies and my own custom 316 gr. cast S.W.C. bullets, out beyond 300 yards. But with either of these bullets, recoil becomes really fierce at the high power end.
