How much do you really gain with a 338/300 RUM over the standard 338RUM in a 28" tube?
The two cartridges give the same performance so that is not your decision. You are not going to gain anything over the other in any length barrel. When the 338 RUM came out in 2001 I tested it head to head with several different barrels with the 338-300 RUM I had been doing since 1998. Same thing performance wise concerning accuracy or velocity.
Your decision is do you want a wildcat or factory cartridge. After the 338 RUM came out in 2001 and guys could shoot both at my range everyone went with the 338 RUM. No need the expense and trouble of wildcatting when a factory cartridge does the same thing.
Is your rum a factory rifle? What is your load with R-25? I jst thought the 300 may be too much for the smaller taper factory rifle.My 28 inch barreled 338 edge is shooting 300 gr SMK 2880fps well under 1/2 MOA with no pressure signs. I use RL 25 powder.My 26 inch barreled 338 ultra shoots the same bullet at 2750 fps.