This is an old post of mine from a few years ago about breech plug fitting......thought it might be useful here.
Wanted to make a couple of comments about breech plug fit.....
1. First and foremost, yes, a breech plug should be well fitted
2. In my business (rocket engines) I routinely seal helium and hydrogen systems (MUCH more difficult to do than powder gasses) up to 10,000 psi and other types of gas systems up to 35,000 psi. I think I am familiar with sealing high pressure, and often very hot gasses (in excess of 5500 degrees F). Anyone who thinks that even a well fitted, straight thread plug with an excellent interference fit on a well machined shoulder can seal hot gas at 10,000 psi is....well... not well informed. No one worth his salt in my business would design a metallic seal like that with any hope of success.
3. The many breech plugs that don't appear to leak hot gas during shooting don't leak for a few very specific reasons.....
a) The pressure pulse created in the bore during firing is very short...i.e. milliseconds....about 3.6 milliseconds for a 48 inch barrel ...and the pressure is dropping the
whole time.
b) The threads themselves form what is often referred to as a "labyrinth seal"...i.e. every time the gas tries to go around a corner, or through a very restricted space, it
drops in pressure. The same as putting a restricting orifice in a gas line to choke down the flow rate.
c) The first bit of powder fouling that gets driven into any leak path around the breech shoulder and first couple of threads forms a packing that enhances the "labyrinth"
effectiveness.
d) NO flintlock breech is water or gas tight....we all purposely drill a hole in the breech area that lets fire in and gas out. Without a toothpick, they all leak water.
e) Every time I fire my much admired Ferguson rifle, the gas leakage around the vertical screw breech blows my hat off....
...no harm no foul. It sure isn't water tight.
Now none of this is to say that a poorly fitted breech plug is a good idea or that one should not try ones best to fit a plug properly, but don't get too carried away with fear and superstition about the effectiveness of the seal between the plug and the bore. The Blue Locktite option is an excellent way to enhance the seal between the plug and the female threads, prevent corrosion, and make sure that if you ever need to pull the breech plug, it will come out cleanly. And for those who think this option is not HC or PC .....or whatever....don't pull the plug on most original rifles. The course threads and short engagement will scare you to death.
Sorry.....this may be more like my 3 cents on this subject rather than just 2.....
