Breech plug threads

[russellshaffer]

Is there any reason not to use a coarse threaded breech plug? 7\8 - 9 to be exact. It's going in a shotgun barrel, not intended for heavy rifle loads.

 

[Kansas Jake]

Good question Russell. I am curious too as in the future I have a shotgun project planned with a barrel that I purchased many years ago. My barrel is 12 gauge.

 

[Col. Batguano]

For inspecting high pressure scuba cylinders, the standard for them to pass is 8 unbroken threads. They are manufactured with 10, so a couple of them could be nicked and it would still pass. The real deal though is the amount of bearing surface between the 2 parties to the joint. Most home shop tap & die sets create joints with a 50-60% bearing surface. A real machinist can create you a joint with upwards of 80%.

I think there's something else in play here too. With 12L14 steel it's fairly maleable stuff. My theory is that when the load is touched off, the plug is forced back some, and the bearing surfaces that are making contact deform a little, so the non bearing surfaces can then come in to engagement, and thus provide some braking action to the recoiling plug. If enough braking action is applied, the plug is stopped. If there isn't enough to stop it before the engaged threads fail, (on either the plug or the barrel threads) then the plug will continue rearwards out the back end of the gun, making for a pretty tough day at the range.

 

[Zonie]

Is there any reason not to use a coarse threaded breech plug? 7\8 - 9 to be exact. It's going in a shotgun barrel, not intended for heavy rifle loads.

I'll assume you are threading a modern 12 guage cartridge barrel?
If so, the chamber diameter should be .813±.002" at the rear and .800" diameter at the front.

A internal 7/8-9-2B UNC thread has a major diameter of .875 min." and a minor diameter of .755-.778.

A internal 7/8-14-2B thread has a major diameter of .875 min. and a minor diameter of 798-.814.

Notice, the minor diameter of the fine 7/8-14 thread's minimum size is smaller than the 12 guage chamber but it's nominal minor diameter size is within the thread requirements.
The minor diameter of the course thread is considerably less than the 12 guage chamber size. If it is used in the 12 guage chamber it will result in a lack of thread surface being engaged with the male thread. This will weaken the thread strength.

On a more general note, the fine pitch threads have larger tensile stress areas and their larger minor diameter provides a larger sheer stress strength than the course pitch threads.

I advise using the fine pitch thread if it is possible.

If you are threading a un-breeched 12 guage barrel, the fine pitch threads higher strengths make it a better choice but either thread pitch should be capable of withstanding the breech pressures in a 12 guage barrel.

 

[Kansas Jake]

In my case the barrel is in chambered. I bought it years ago from Numerich, or Dixie but I can’t remember for sure.

 

[russellshaffer]

My barrel is unchambered. The ID is very close to the recommended diameter for a 7/8 - 9 tap, only about .04 under. I figure that will give just a little extra bite for the breech plug thread. I will have to make the plug so I can have a full inch of thread engagement to work with. I'll make the threads a tight fit with as little slop as possible. Steel is cheap so I can make as many plugs as I need to get one just right. I may get it done in 2019 with luck.

 

[Zonie]

For what it's worth, the nut or internal thread doesn't have to be any longer than 3/4 of the thread diameter to give all of the strength the thread is capable of delivering. Put another way, making the thread length longer than it needs to be doesn't give the thread any more strength.

For your 7/8 inch thread, that results in a thread length of .656 or, 9/16" for a ball park figure.

Because the vent hole or the hole from the drum must be in front of the breech plug face, using this shorter breech plug length should help to keep the architecture of the lock and barrel looking more traditional.

 

[Kansas Jake]

Thanks Zonie. Also, my barrel is unchambered too. My fingers must have gotten ahead of my brain or spellcheck zinged me.

 

[Col. Batguano]

That's interesting. Most plugs today are threaded 1/2" long, which by that metric, is overkill for any caliber less than 66 caliber.

If you asked them, would the barrel makers make you a threaded breach / plug that's shorter than what is otherwise "standard"? I think Charles Burton at FCI might, (or probably could) but I also think he uses "standard" pre-made plugs for his barrels.