I may be seeing your problem "backwards" in my mind's eye ....... but it sounds like the barrel overlaps the frame?? If that is the case,
I would think the barrel/arbor fit is the problem. Addressing a "too deep" hole in the bore for the arbor may be a much simpler
solution to the problem.
If you lap the frame to move the back, you may well adversely effect the Barrel to Cylinder gap. If I'm being the problem correctly, I
would suggest dropping a brass flat washer down the bore and see what the difference is. If the washer opens the spacing at the front of the frame slightly, the washer can be surfaced to fit. The washer can be attached to the end of the arbor with a dab of JB Weld and
then the washer dressed to fit.
Or ....... I'm not seeing it correctly and you can ignore the above and return to our regularly scheduled wild a$$ guesses
Coffinmaker