Snail vs drum ??

[Tumblernotch]

Does the drum really have to be supported by the lock plate? The reason I ask is that on most flintlock conversions there is a large gap below the drum- wouldn't some sort of shim been put in there?

It certainly wouldn't hurt to have the added support of the lockplate, especially if the gun has a very strong mainspring. Under normal conditions there shouldn't be a problem, but you have to take into consideration that the threaded portion of the drum could be weakened by corrosion, especially on an old gun. And if there is a strong mainspring, that drum is taking a beating at its weakest point which is the threaded shank.

 

[Zonie]

There may be many flintlock conversions where the drum is not supported by the lockplate but I don't know why this should be.

This picture shows a Tennessee style rifle I built which is using a Large Siler flintlock which I converted to Percussion*.

Notice that the drum is supported by the lockplate. I even left a small portion of the flintlocks pan standing proud of the face of the lockplate to extend the support to the drum out further than a normal percussion lock.

Because the top of the lockplate is usually at the center of the barrel there is more than enough material to use to file the circular support for the drum.
The only thing that actually limits the size of the drum is the underside of the bolster which the mainspring pushes against.
Obviously the cut cannot be so large that it will cut thru this area.

* Why would I convert a Siler Flintlock to percussion?
This is a Siler flintlock kit. When I drilled the hole for the frizzen screw thru the lockplate and frizzen something slipped and the drill ended up breaking out of the side of the frizzen bridle.
Rather than just cutting off the bridle I decided my armory needed a real "converted Percussion" so I finished the lock with that in mind.