My response was based on information I have gained from two customers suing each caliber.
THe 338 RUM was running just over 3300 fps with the 200 gr Ballistic Tip a seperate rifle in 338-300 RUM which I did not build but did some stock work on was shooting in the 3360 fps range with what seemed to be similiar pressures.
This held true for bullets such as the 180 gr Ballistic Tip where the velocities were roughly 3450 and 3500 fps for the two rounds.
Again, I do not knwo if these are full tilt top velocities as they were the accuracy loads out of each rifle with these bullets.
The 338 was a factory Rem 700 ss which I restocked with a laminated thumbhole and pillar bedded along with an accurizing job and installation of a Holland QD brake.
The other was a second hand custom rifle
of unknown origin but was built to very quality specs except the neck and throat were of SAAMI dimensions so this may be the reasoning for the little amount of velocity gain.
This is a rough estimate on performanc eof each round but I believe an accurate on average assesment. As always some comparisions may have a slightly larger variance, some smaller.