Any reason to shorten plug length on patent breech?

[HighUintas]

I have a GM barrel and waiting on a Griffith breech. I tried to find out what length that breech plug journal length is today but couldn't get anyone on the phone.

I had thought I would shorten the GM breech to 1/2" from the 5/8" that it currently is, but then got to thinking.... Is there any reason to do that unless the plug journal length is 1/2"?

So... Is there any good reason to shorten the breech length on a Hawken percussion breech?

[Leatherbark]

Disregard if you are already aware of this but, I would not shorten it yet. A patent breech is much easier to install on a new barrel as you can clock it to the next available flat by only having to file the thread shank shorter.  If you are installing it on a barrel that already has underlugs and sight dovetails, you must time the end of the breech plug and the octagon section to close at the same time on the barrel flat that you have to use.  Of course, if you are using a lathe, you can place it on any flat you want much easier.

For what it is worth a standard TC type breech as sold by T.O.W will tighten up at both ends in a new GM barrel with little filing on the threaded end, allowing one to pick the next available flat so the breech face will tighten at the same time the octagon flat closes up tightly.  The only drawback is that sometimes the "GM" with the little Mountain trademark will be on a visible flat.

[rich pierce]

I can’t think of a reason to shorten it on a patent breech.

[HighUintas]

Thank you both. I'm glad I haven't shortened the barrel yet and hopefully I won't need to for the breech plug. I guess I just had in my brain the idea of the breech plug needing to be a little bit shorter when building a flint lock for better Lock replacement and carried it over to building a percussion, even though I didn't know the exact reason behind it in this context.

[B.Habermehl]

I normally acquire a bottom tap for any barrel that is supplied without a breech plug like GM. And be certain the threads continue to the shoulder in the barrel. Then fit any breech plug. BJH

[Jim Kibler]

I'll also add that I've found it beneficial to even grind down a bottoming tap further to help get the threads closer to the shoulder.  You can't go too far and still need to have a little lead on the tap, but it can generally be decreased.

[HighUintas]

Thank you both! I had thought I'd probably get a bottoming tap for this reason, but was going to wait to see if it is needed. I believe the plug journal has a fairly long portion at the front end that is relieved of threads, at least that's what I see in pictures, so it may not be needed.