As a general rule of thumb, we always used 1/2" deep plugs, and , fortunately, never had a problem. I am not an engineer, a trained tool and die maker, or even a trained machinist. I had on the job training. When we first bought the
Paris barrel business, they had us making coarse thread plugs.....3/4-10, 5/8-11 threads, similar to the old barrels. I don't
think there are many people on this forum that have ever done these, but, it is virtually impossible to make them
waterproof. When you look at these, the threads are big, and apparently will stand the pressures present in a muzzleloading gun. One other thing aboout these coarse threads....we chased all of the breech plugs on a lathe....very
time consuming. After several months in the barrel business, we went to the fine threads....5/89-18, 3/4-16, etc., mainly due to pressure from certain individuals whom I choose not to mention at this time. We had an old turret lathe
which we brought along from the Paris shop, but never used it. We bought a threading head and now use it to thread
breech plug blanks in the fine threads.....sure speeded up the operation. We have always stuck with the 1/2" deep threads, regardless of the size of the plug....5/8", 3/4" or 7/8", unless we were fitting a patent breech, which normally
came with a longer threaded shank. We have done a lot of proofing of barrels and never had a plug blown out of the barrel. I think you could possibly do shorter threaded breech plugs, assuming it was done with extreme care. When you
are working in a somewhat production mode, you cannot take the time to "fit" every plug.....using layout fluid to se where
you are hitting, correct it and try again, etc......talk about a way to lose money. John Bivins used to ask what sore of
"crunch" factor we used. One cannot describe this, but, you make a plug, threaded, and screw in the round blank, using
a 14" pipe wrench and you actually crunch the plug against the bottom of the threaded breech...done. That is what we
called fitting. I'm sure all of the other companies do the same thing. So there you have it..............Don