Bob, I've gotten interested in the 404 and have a No. 1H chambered for it. It is a very interesting classical rifle, but like you, I see the 404 Rimless NE as a magazine rifle round. I found a M-70 Classic (SS), now waiting for conversion, as I think that's the optimal modern starting point. The feed rails and follower are already optimized for the Jeffery case and the bolt face is very close, so all that will save some work.
Comments are based on having loaded the cartridge with 400 grain bullets to the 2300 fps range and 450 grain Woodleigh solids to the 2200+ range, which is a significant improvement over the 2125 fps/400 grain MV original specs that made the round's reputation as a DG (elephant) cartridge. Although I believe Ray's assertions about 2600 fps MV, it'll come at a very significant cost in recoil - that's 416 Wby territory - and probably represents lots of KE expended on the off side of just about anything short of body shots on ele.
The 404 Jeffery Hornady brass I've gotten seems uniform and, if it's manufactured to the same specs as the 375 & 416 Ruger cases (which I've reloaded many times in the 404-375 Ruger at these same MVs with the .423" diameter bullets), should prove strong and last many reloads. I haven't shot the 404 Hornady Jeff cases yet, so can't comment on their durability specifically.
In a reasonable weight rifle (9 - 10 lbs) recoil becomes very noticeable in the 404s above 2300 fps IME. This round's forte is balance - significant power and moderate recoil in a trim and lively rifle IMO. All the nostalgia of the 375 H&H and ample power for anything to boot. Carpet diem, Bob, go for it!
