Also...
The actual length of the plug might not be 3/8" long. It might need to be longer. It all depends on the depth of the ramrod hole. But you need about 3/8" where the screw will be. Sorry if that causes any confusion.
Now I'm confused...
If the plug and screw causes the ramrod to stick out the front... wouldn't the lug and pin do exactly the same? I mean, the screw is going in right around the same spot as the lug. Both would block the ramrod hole.
The lug/screw should end up somewhere below the lug on the back of the breech plug. If the ramrod hole is this deep, it was probably drilled too deep. The ramrod hole doesn't need to be any deeper than the length of the barrel. Then, when placed in the bore, the ramrod should stick out about 1/2"-5/8"... which is the length of the threads on the breech plug.