Breach plug length

[Dphariss]

With a bore diameter of .540 [ 54 caliber]  The maximum area of the breach plug is .229"  if you had a chamber pressure of 10000 psi. , the pressure on the plug would be 2290 lbs.
    The pressure on the first two inches of the barrel would be more than 16964 lbs   That is about a 8:1 ratio. 
  Numerous engineers have told me that a full three threads, with a standard fit,  provides maximum strength. Therefore, it is just a matter of looking up the tensile strength of the type steel used.  However, for all practical purposes if you have a sufficient amount of threads the barrel is 8 times more likely to fail than the breach plug.  If I'm wrong on this , somebody prove it.
   There is another factor in this. It is not possible for us common small shop operators to make a plug that has threads for it's full length. Close to the base there is a short section about 1/16" to 1/8" long that is not threaded. even on a lathe this is hard to do. this must be compensated for. So--- it's not the length of the plug that is important, it is the fit, the length of the threads and the type of steel that is important. The steel in the plug should be equal to the steel in the barrel in tensile strength.  Right? Right.

Your PSI thing is correct and one reason I like to set my breech plugs and vent liners to a shoulder.
I tend to cut breech and vent liner threads with a split die that is opened up to make a tight fit with the tap used. If too tight I tighten up the die by backing off the screw a *very little* and recut. I do this right in the lathe after getting the shank diameter correct. Using the tailstock to hold the die square with the shank, or one can cut the threads partly with the lathe. This will allow threading right to the end or very close to it if the die is reversed for the second cut and the threads will "fit" the threaded hole.

Dan

[Acer Saccharum]

OK....  so it looks like I really open up a can of worms 

Don't worry about the worms, we like 'em. This is a worthy topic, and as you can see, almost everyone has something to say about it. You also see a variety in approaches to a problem, different ways to 'skin the cat', and often find something that suits your abilities and tools.

I often spout off about my process, and someone will politely point out a flaw in my logic; this is a good way to learn.

Acer

[B Shipman]

Remember we're talking about a newbie here. There's no way with 30 yrs experience I can cut a 3/8th plug and come up with, after fitting, a 3/8th in. plug. Shoot for at least 1/2 in.
 My concept of mechanical engineering is that a plug reaches maximum strenth when length equals width.  However , this may not be necessary. But the bigger the caliber , the longer the breech. In theory.

Don's statement about the jam fitting of barrels is extremely important. I doubt a  Getz barrel has ever failed in this area. Rice barrels are similar. Now these barrels are cut full depth. But newbie is not using one of these.
Now let's talk about Green Mtn. barrels, and yes, Douglas, and others. They are not even fully  cut to the shoulder.
There's no way you can fit a plug properly with what you're given.  You need to get the best possible bottoming tap from Brownells or someone, grind the face to
full threads and recut it. What you get will not do.
In reality, this is what the newbie is up against.  Trying for a 3/8th in. plug is ridiculous.
What is perfect and what someone can do for the first time is vastly different.
$#*!, if I were him I'd be shooting for 5/8th in. I quarantee it'll be a lot less than that in the end.

[bbhf]

OK: i have this thread and am still confused: i have a pre carved stock and green mt barrel from pecatonica : the barrel is 42" 50cal, 7/8" dia: the flared breech plug is 5/8" long: i have read a lot about a 1/2" plug: after reading this thread should i leave it alone or cut it back to 1/2"? if i do cut, how should i go about it: i can have it done in a shop if need be, or can i cut, file, tap and fit? will the difference in the 1/2 and 5/8 play a big role in the trigger and lock placement?  do the green mt barrels have run out? any help would be appreciated: bbhf

[J.D.]

 It is not possible for us common small shop operators to make a plug that has threads for it's full length. Close to the base there is a short section about 1/16" to 1/8" long that is not threaded. even on a lathe this is hard to do. this must be compensated for. So--- it's not the length of the plug that is important, it is the fit, the length of the threads and the type of steel that is important. The steel in the plug should be equal to the steel in the barrel in tensile strength.  Right? Right.

Yep, and don't forget the tapered threads right behind the face of the plug. In most cases I have seen, there is at least 1/8 inch of partial to no thread engagement. So, in a 5/8" long plug, you will have roughly 1/4" of no thread, or partial thread. That leaves 3/8 of fully engaged threads.

To reiterate what Jerry said, it's the fit and length of fully engaged threads, in addition to the type of steel, that is important.

OK: i have this thread and am still confused: i have a pre carved stock and green mt barrel from pecatonica : the barrel is 42" 50cal, 7/8" dia: the flared breech plug is 5/8" long: i have read a lot about a 1/2" plug: after reading this thread should i leave it alone or cut it back to 1/2"? if i do cut, how should i go about it: i can have it done in a shop if need be, or can i cut, file, tap and fit? will the difference in the 1/2 and 5/8 play a big role in the trigger and lock placement?  do the green mt barrels have run out? any help would be appreciated: bbhf

Follow the advise given above on getting the correct bottoming tap from Brownells, and leave the plug full length. The longer threads do not interfere with lock or trigger placement, so there is no worry there. Just center the pan on the location for the TH.

I'm not aware of any runout on GM barrels.


As to blown and ringed barrels, I have not personally seen a blown barrel, but I have seen a coupla ringed barrels. The plugs were intact, in those two cases of ringed barrels. One of them was a Bill Large barrel that I could have bought for a song. Maybe the owner knew it was ringed, dunno, but the song turned out to be a sad one.

God bless

[bbhf]

J. D. thanks: i will follow the advice and also use the blue to see if the plug is seating or not: after i posted this i was looking through tracks catalog where it states that the green mt barrels are ground on centers: skip

[Dphariss]

J. D. thanks: i will follow the advice and also use the blue to see if the plug is seating or not: after i posted this i was looking through tracks catalog where it states that the green mt barrels are ground on centers: skip

The straight barrels may have some runout.
This is one way to put the vent behind the face of the breech.

Dan

[Daryl]

Properly threaded, I'd have no trouble with Dan's breech above.

[rich pierce]

Just gotta hope somebody down the line who doesn't know how it's constructed, doesnt try to remove the breechplug w/o removing the liner.

[bbhf]

Thanks for additional info: i will have to see how everything lines up and where the lock will sit : since this is the first rifle to be built i am hoping to avoid an "oops" that can't be corrected: skip

[Dphariss]

Just gotta hope somebody down the line who doesn't know how it's constructed, doesnt try to remove the breechplug w/o removing the liner.

There is a saying that a potter loves the sound of breaking pottery.
 
People were agonizing over the pan fence/breech problem this is one answer.
I would rather have this breech than a sliver of wood running up from the breech to the fence. Looks sloppy. Don't care if some originals look like this, still looks like a mistake was made.

Really there is little need for pulling a breech and a vent liner 1/2" on center from the breech of the barrel surely has to intersect the plug.
The other solution is a Nock type breech.

Dan