I don't have a vertical mill that is practical for the job, but I have a bench mounted drill press that is. I spot glue my rib to the barrel, if it's one of the solid types, set the barrel in the vise so the bore is level, set the quill stop so that I don't drill into the bore, and have at it. I drill first with the tap drill right to full depth through the rib and into the barrel. Then I give the rib a crack on one end to break the glue (Cyanoacrylate), drill the clearance holes and countersink for 6 x 48 scope mounting screws with the rib off the barrel, and return the barrel to the vise to use the drill press with the belt removed to tap the holes. Before I had a compressor I used an aerosol can of WD 40 to blow out the chips during the tapping.
I should say I used to do it that way, because now, I solder them all on.