I thought about 338 rcm because I did read about the issues with Winchester not being able to achieve the same performance of a LA cartridge in that small of a package and ruger touted theirs as being able to do that and from a short barrel.
Unfortunately, when the brass and loaded ammunition hit the market there were lots of folks ready to test Ruger's claims. The end result you see now, the .338 RCM is but a ghost in the background due to the absurd claims made by the developer. Ten grains less powder and shorter barrels, yeah right... nice idea bad development.
I got one of the first test batches of the .338 WSM a long time ago. I even got the RCBS dies at the same time. My use was intended for short actions, 2.825" OAL and DBM fit at 2.925" OAL. Obviously this didn't include those VLDs since it forced the bullet back into the case, leaving less powder capacity. But the reality dawned where this cartridge could shine in the area of, as you state, 25 - 350 yards. No need for those arrow-like bullets when the older cup and core designs would shine.
Nosler Partitions, Woodleigh PP and now the newer Nosler E-Tip provide plenty of energy from the 225 gr. versions. Unfortunately, even the heavier designs will not generate 2 tons (4,000 ft/lbs) even at the muzzle. My loads will come in at 3,700 - 3,800 ft/lbs
at the muzzle leaving 2750 ft/lbs at 300 yards. No magic pixie dust required just good old fashioned Re16/17 in good brass. I generally use the Winchester 325 WSM brass and neck up to hold the .338 bullets. I do a basic sort because it is Winchester after all, then do a regular brass prep.
The barrel I started with was a reject from another member because it was too short. I thought it might provide a nice woods length carbine for the .338 WSM test bed. And yes, 21" is good enough to get the numbers I need and allow for all the powder to burn inside the barrel. This might seem trivial at first but picking the right powder will have a definitive effect on your shots taken dawn and dusk. Super heated plasma exiting the muzzle allows for a larger than normal muzzle blast. Try that when shooting on the fringe of legal light!
Enjoy the process!