I find it hard to believe that a JR press would not size standard rifle cartridges like the .30-06 or the .308 effectively.
.30-06, .270, .308, .303 British, 8 mm Mauser, and really anything in the standard rifle cartridge class should work just fine.
Likewise with the C presses of the 50s, 60s and 70s.
These were the bread and butter cartridges that people bought these presses for, and the cartridges that RCBS designed the JR press for. The changeover to the O press was because C presses tended to flex with rifle case resizing.
Sizing .303 British is not difficult at all, so I really can't see why .30-06 would really be more difficult.
Certainly a compound leverage press would be easier though.
I own a Redding Big Boss II which is superior to the much older Rock Chucker design, but I got by without it for many years when the JR3 was my only press.
I never felt handicapped.