I have a RPR .308 20". Only shot a little. Got it from a member.
One very very important thing to look at: Why is there no magnum American or Precision?
The Ruger American/Precision has a 1" barrel shank, with a ~.94 minor diameter.
With a SAMMI chamber major diameter of .5563 there will be thin metal for a magnum around the chamber to contain the ~65kpsi.
.94 - .5563 = 0.3837
0.3837 / 2 = 0.19185
Compare that to a "standard" barrel shank with a 1 1/16 barrel shank with a .98 minor diameter.
.98 - .5563 = .4237
.4237 / 2 = .21185
.21185 - .19185 = .020 more metal around the chamber. In reality this is probably more like .030 ....
Yes there is a possibility I have my dimensions wrong. Ive not made these threads and I'm not don't with my coffee. I went off a thread chart. If you know more/better don't bitch, fix it.
Doesn't sound like much but Ruger chose not to make a magnum (yet).