When I converted my T-C Hawken from flint to percussion I hadn't got my lathe yet so I bought an undrilled drum that was threaded to fit the flash hole in my barrel. I screwed the drum into the flash hole, snugged it up, then filed the clearance in the lock plate so the barrel would drop into the stock.
Then I carefully allowed the hammer to rest on the drum, using a sharpie to mark the outline and center of the hammer face on the drum and a straight edge to mark the end of the drum for the angle of the hammer face. I set up a vee block under the drill on my drill press so I got as accurate a hole into the center of the drum as I could. Then I reamed it with a tap drill and tapped it. After I got the drum back in the barrel and the nipple in the drum, everything lined up fine. All this seemingly took less time to do than describe.
I haven't had a misfire since I got a nipple with the right length threaded portion. ([size=75]knock wood[/size]). A drum requires a much shorter threaded part of the nipple than a snail bolster.
Three Hawks