Finally had this out at the range yesterday to try my attempts at loading for this rifle and the 500 BPE.
Casting showed a fairly tight bore at 0.504. Case is 3" to leade, 3.36" to rifling, which is standard 6 groove with lands about 0.09" wide and grooves 0.171" wide.
Load was 137gr FFg Old Eynsford and a unpatched .500 370gr cast bullet. It's on the paper at 50 yards, but not regulating. From Graeme Wright's book, I believe I have too low of velocity. Since there's no room left in the case, I'm guessing this means trying 1) a lighter projectile, 2) FFFg, or 3) paper patching what I have now with the 370gr cast bullet for a tighter seal and less friction.
I'm planning on loading 6 more with my original try, since I was a bit anxious about shooting this the first time (first time ever shooting a double rifle!), plus 6 paper patched, with same charge, then 6 patched with FFFg.
Any other suggestions? I'd like to see if I can get these 370s to group before trying a different bullet.