: Upon thinking about the rim size, it is possible the Ruger may need some milling of the trough, however, it works fine with the .608" rim of the .45/70. Due to the taper of the .500 case allowing some angle to loading a round past the trough, it's rim might clear OK with no modification. The only way to find out is to chamber the round and test for fit.
; Pat, there is no limit to OAL, however the longer the round, the straighter it must be in line to the axis of the bore when chambering it and the lower the rear trough must be to clear.
: Donnley's book says the .500 3" and 3.25" (alone with the .500/.450) rim is .660" in diameter. This is only .058 larger than the .45/70's rim. That is only .026" per side, which means a .45/70 Ruger, if different from the rest, may have to be milled just a tich. I had to mill one Shilo Sharps factory action to clear the .50 3-1/4" rim of .656" while the other one we re-barreled was fine without milling.
; .026" is about 4 thicknesses of 24 pound in-jet printing paper. Only the centre radius must be that deep as the rim is round. It could be done with an emery stick in a hand drill using 320 grit emery and finished with 400grit before blueing.(action apart, of course)