Breech Plug Thread Length

[t.caster]

I am about to install the breech plug in a .58 cal NOS Numrich barrel. I bought a 3/4-16 flared tang plug with threads length of 3/4". The barrel is threaded .780 deep, so some filing is necessary of course. I am inclined to use the .750 depth, but that moves the touchhole liner 7/8" from the end of the barrel or more (thinking lock placement).
Peter Alexander's book says breech threads don't need to be longer than 1/2"deep, but that doesn't jive with my machinery handbook and years in the structural steel and automated machinery design training. Maybe he is thinking of smaller caliber barrels in his statements.
I would just like to hear from some experienced builders before cutting anything.

Thank You

Tom C

[D. Taylor Sapergia]

I would cut the breech plug to 1/2" of threaded journal, and the barrel to match.

I have an original Staudenhammer double shotgun whose breech threads are as new, and they are 3/8" long...14 gauge.

Taylor

[R Whittington]

I never use more than 1/2". I've never had a problem.

[smylee grouch]

I think most of todays barrel makers use 1/2 inch but Green Mountain used to use 5/8 inch on some barrels, don't know if that is the case on all GM barrels and any current made ones.

[FDR]

I am about to install the breech plug in a .58 cal NOS Numrich barrel. I bought a 3/4-16 flared tang plug with threads length of 3/4". The barrel is threaded .780 deep, so some filing is necessary of course. I am inclined to use the .750 depth, but that moves the touchhole liner 7/8" from the end of the barrel or more (thinking lock placement).
Peter Alexander's book says breech threads don't need to be longer than 1/2"deep, but that doesn't jive with my machinery handbook and years in the structural steel and automated machinery design training. Maybe he is thinking of smaller caliber barrels in his statements.
I would just like to hear from some experienced builders before cutting anything.

Thank You

Tom C

I was taught the same as you. 1 and 1/2 diameters of thread for max strength.

[sqrldog]

Thousands of barrels have been breeched .500 deep. I figure if many had failed they would have made them deeper.  I have even breeched some a little shorter to get proper lock placement.  So far they've all worked fine. I think in most instances barrels correctly breeched 1/2 deep are safe, but do as you think prudent. Tim

[Ky-Flinter]

Rice breeches all their barrels at .500".  I just finished breeching a NOS Douglas .45.  It was threaded 5/8" deep, now it's .500.

-Ron

[L. Akers]

I recall that for maximum strength the threaded portion should be as long as the major diameter.  That being said, I would agree with the opinions expressed that 1/2" length is plenty.  I make my plugs to agree with the lock am using -- that is the distance from the rear of the fence to the center of the pan.  That puts the lock in the proper place.  I un-breeched an 1858 Springfield for a fellow and its plug was 1/2" long with a very coarse thread.

[David R. Pennington]

I just did a NOS .45 barrel from 1973 that was breached for a real coarse thread (9/16 - 12) and I decided to err on the side of caution and left threads full depth. Wish I had cut it back to 1/2" now. Lock placement is hard. Now with lock farther forward correct placement of double set triggers will require tang bolt to be further forward in tang then I would like.

[Bob Roller]

Thousands of barrels have been breeched .500 deep. I figure if many had failed they would have made them deeper.  I have even breeched some a little shorter to get proper lock placement.  So far they've all worked fine. I think in most instances barrels correctly breeched 1/2 deep are safe, but do as you think prudent. Tim

The old rule of "thumb" was if a 3/4 inch diameter is needed then the length should also be 3/4" deep.I think that if the threads are proper depth in the barrel and the plug diameter is right then a shorter thread would be OK.
One thing is sure,a ball or bullet will move forward much easier than a properly installed breech plug will move backward.

Bob Roller

[t.caster]

Thanks, I will go with 1/2".
The 1 to 1 1/2 rule of thumb on structural bolts is for course threads and fine threads result in more yield strength.

[Herb]

You seem to be a machinist and know how to fit this plug, but there are people who do not have your skills or machinery and still want to fit a plug.  I did a tutorial on "Fitting a Hawken Breech Plug by Hand" in the Tutorial Metal Working Section.  Of course, it works for other plugs, too.

[James Wilson Everett]

Guys,

Just an interesting observation, please not an argument.  A standard nut has only 3 UNC threads, yet if the bolt and nut are of the same material then tightening to failure results in a failed bolt shank, not stripped threads.  Only three threads.  I am not advocating breeching at three threads!!  But threads are quite strong.  I use a thread depth of slightly more than 0.5 inch, which gives about 8 full threads with 18th c tooling.  Those above advocating 0.5 inch breech plug thread engagement still have a very high safety factor at the breech.

Perhaps more important is the quality and fit of the threads - make them good!  Also, the material of the breech plugs - cast plugs are suspect, at least by myself.

Jim

[t.caster]

Everything is cut and trimmed and fit at .500 +/-. Got a tight fit on both ends.
Good to go!
Thanks for your reassurances.