that's not a full drawing of the trigger mechanism... is it?
No
UC, the drawing only shows the three main contact points, not all of the details.
How about insetting the breach end of the barrel about a quarter inch and add an alignment mechanism there?
I want the barrel against a flat face so I can use a variety of barrel profiles and sizes. If it works as I hope I expect to have 1" and 15/16" octagon barrels, probably a round barrel or two, and even 20 and 12 gage shotgun barrels.
I would also use a larger diameter mating bolt if at all possible.
Yeah. That's one feature that worries me a little. I went with the quarter-inch for two reasons; there needs to be enough clearance between the hex head of the bolt and the top of the breech block to get a wrench on the bolt, and the quarter-inch bolt better allows for smaller caliber barrels. It can always be modified if early testing indicates a larger mounting bolt is called for.