Well, now, with a percussion lock, you are going to be using a drum, I presume. The lock can really only pivot around the drum. When out of the stock, you hold the lock into its drum on the bbl, and rotate it to see what angles you can get, and how much space you have under the bbl for a bolt. If you need to rotate the lock Clockwise far enough to miss the ramrod, it may be at a real dumb looking angle. You need to pursue alternatives.
Take some modeling clay, and you can stick the lock in place on its drum in various positions. The clay will hold enough to allow you to turn the assembly over for measuring. You can even mock up a piece of ramrod, and stick that with clay, replicating the web you have.
To find your exact web thickness, drill a little hole from the barrel channel into the rr hole. then you can use a small pin to measure how far down that is.
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