Thanks for the additional info!The Federal Safari Trophy Bonded Bear Claw in 400gr also claims 2250 fps. I tried the 404gr Shock Hammer from Aria Ballistic Engineering but was only able to get 2250 out of my 23” McGowen barrel even though they listed 2500 fps as their rating.
If the two Ruger offerings had been more widely successful, then I think necking up the 416 to a 458 & necking down the 375 to something a bit smaller could have produced a useful series of chamberings for standard sized actions.
The various PRC cartridges are all based on the .375 Ruger case and seem to be taking off to some extent, so hopefully that will help keep the .375 and .416 alive. I think they'll live as long as the Ruger rifles chambered for them are available, then fade somewhat. There are probably quite a few M77's out there, particularly in .375, so it will be a while before they become orphans.
The RCM's were also based off of a shortened .375 Ruger case, but they're pretty much entirely dead at this point.