Sabotloader - just walked to the shop and brought back the rifle.
My lock is hard against the barrel. This lock does not have a half moon dip in it. Instead, both the original barrel and the GM have a slot in the underside of the bolster to allow the straight top edge of the lock to pass "thru" the underside of the bolster.
What I do notice is there is a hair of travel permitted by the wedge, and when I allow the barrel to travel as far back as the wedge pin allows, the hammer will clear the nipple. Don't know if it will clear a cap, but does suggest I may be able to remove a hair of material off the tang face allowing the barrel to come back that hair.
I also notice the hook of the breech lays a bit to the left of center where the original barrel hook is dead centre, suggesting all of the material I take from the interior of the tang will come from the one (left) side.
If the bolster/breech plug of the GM barrel is fitting properly in the drepression on the lock then alignment can be had...
Given my description, still think so?
The barrel needs to move left (from above) to center the nipple. The harder the barrel lies against the lock the less clearance the hammer has and if I press it hard against the lock the hammer lip hits the nipple spout - doesn't clear cleanly.