The 338 Lapua Mag was designed by Sako (rifle), and Lapua (bullet development), and both companies are Finnish. The concept was originally brought out by the Navy Seals using the 416 Rigby case. The case couldn't handle the pressures that this round produced and so the Seals dropped the project and Sako/Lapua picked up the concept and developed it. Sako produces the TRG-42 and did produce the TRG-S hunting rifle, Accuracy International does, Armalite does, PDW does, and several other companies I have heard besides Gunsmiths.
Here is some data from Lapua:
250 gr SP or 250 gr Scenar match HPBT:
Velocity, 0 Mtr 900 M/S, 600 Mtr 633 M/S, 1000 Mtr 481 M/S
Energy, 0 Mtr 6561 J, 600 Mtr 3245 J, 1000 Mtr 1874 J
(the data is in metric)
The 338 Lapua excells with the 300 gr match HPBT bullet as compared to the other calibers mentioned, I am not well versed. I have shot the 300 Rum in a M700 rifle and wasn't impressed with the setup and accuracy. The Rum was a loaner rifle for me to develop a load for a friend, he finally got tired of finding a good load for that rifle and sold it.