I tried an 8mm RM once upon a time and the big case is deceiving, this cartridge does not live up to it's capacity expectations..........I got better velocity from the same bullet weights in a 300 Wby with the same barrel length. I have found the 7 STW, 8mm RM and the 358 STA all to be sub performers given the powder capacity. I believe there is a design flaw with this case and it does not perform as well as it should given the capacity it has. The 7 STW does not do any better, in fact it won't match the 7mm Wby, the 8mm RM won't achieve the same velocity as a 300 WM with 200 gn bullets, and in fact falls 50-100 fps short of the 8X68 imp 30 deg with equal bullet weights and the 358 STA, barely if at all exceeds the 358 NM with equal bullet weights. I have loaded and compared all the cartridges and have found the entire 8mm RM family to be disappointing under achievers. Every one of these calibers out does it's long case cousins, when housed ahead of a 300 WM case, and I'm talking by well over 100 fps per given bullet weight. The 7mmX300 WM out does the STW by 100 fps with 160 Parts, the 8X68S imp 30 deg (identical capacity to the 300 WM) out does the 8mm RM by 100 fps with 200 and 220 gn bullets and the 35X300 WM out does the 358 STA by 110 fps with 250 gn parts.........all 24" barrels and all loaded to ejector marks (just slightly). I have done all these comparisons myself and clocked them all through the same Oehler 35P chrono.
Given their powder capacities every one of these cartridges in this family should do 200 fps more than they will actually do, yet none of them will.............design flaw (shoulder angle I suspect). JMHE........