The "long" moniker wasn't my idea. Some wildcatters used a 6 BR reamer and plunged it approximately 1/2" deeper to make a 6 BR "long" which is actually a 243 with a 6 BR shoulder and neck with less body taper. Called the 6mm BRK.
I did the same thing with a 30 BR reamer making a 308 with 30 BR making a 30 BR "long".
The RPM case is interesting because of the shoulder angle minimal body taper and its rebated rim with standard bolt face diameter of .473". I would bet a 284 reamer could be used cutting deeper and headspacing on the longer case.