The problem with the M77 RSM is that the magazine box is too short for the 505 Gibbs. It is plenty wide enough, just too damned short. The 500 Jeffery and/or 500 AHR feed just fine from the stock Ruger RSM 416 Rigby mag box, albiet the feed ramp needs widened. Also, you will need to drop the bottom about 0.25" to comfortably fit 3 rounds of 505 Gibbs, a little less to get the 500 Jeffery/AHR to fit. This isn't as difficult to do as one might think since the Ruger uses 2 piece bottom metal.

The last major issue is that the Ruger has NO receiver recoil lug. So one must fabicate either:
1.) A recoil plate which is sandwiched between barrel and receiver (cheesy solution looks like a M700)
2.) Fabricate and fit two recoil lugs to the underside of the barrel. This is easy to do if one is reboring the Ruger barrel since it has a second 1/4 rib on the bottom side of the barrel (pretty much like the one on top).

Personally, I think 500grains has the right of it though.

Scott