What's the minimum safe breach plug ?

[Dphariss]

It is interesting that some are calculating what sort of threaded length is required to ensure the breechplug’s hold on the barrel is no more likely to fail than the barrel itself, and using that as a standard. That clearly is a new standard not previously employed in making muzzleloading, black powder-burning barrels. Has something changed?

My thinking is that the explosion is contained by the barrel assembly as a whole, not just the breechplug threads, and the assembly is only as strong as its weakest point. Since the strength barrel walls are a fixed point, I figure that any breechplug that is as strong as the barrel walls is as safe as it can get  - If you have two identical barrels, one breeched with threads that will fail at 55k psi and one that will fail at 100k psi, the 100k breech is no safer than the weaker one if the barrels themselves will come apart at 50k psi.

I don't particularly like the idea the of making a breech that cannot be taken apart to check for corrosion, remove a stuck ball or cleaning jag, de-hex it after missing a shot at a wolf  , etc. That isn't a theoretical concern, either - on my to-do list is disassembling and examining a percussion rifle with a interlocking plug and drum system that hasn't been used or cleaned since it was used to shoot some pyrodex blank loads and cleaned by an unsupervised teenager about fifteen years ago. It is hard enough with a percussion system - I have no idea how I would do it with a flint setup with no way to unscrew the liner. While I don't want to compromise on strength, getting fixated one point and overengineering the breech threads at the expense of other concerns seems like a bad idea.

If it was not cleaned after shooting pyrodex, even for a few days I would not worry about looking at it. Its no longer usable so just plan on replacing it.
I NEVER remove a liner. I learned this LONG ago. Like 40 years ago. If its put together right there is no need. Seating the liner against a shoulder means there is no need and I sure don't want one with a screw driver slot. Not only is it ugly its not needed. Shouldering the liner drastically reduces the pressure on the part making it even safer. Unfortunately most of the store bought liners don't allow this.
Dan

Dan

[Dphariss]

Photo sucks but it shows the breech seated and shows where the vent enters the "cup". The liner is shouldered.

[Dphariss]

This is a barrel maker installed breech, no seal at all and lost of fouling in the fouling trap thus created. The plug threads can be seen if you look past the reduced diameter "face" of the plug.  I replaced the barrel for the guy since it also had deep dovetails in the waste and since it had been in my shop I did not want to be responsible.
Dan

[Dphariss]

It looked like this after I pulled it. Before and after removing the trapped fouling. There is no excuse for this. Its a commercial barrel maker but they were producing hobby home crapsmanship. Maybe still are. This is why I never use a barrel breeched by someone else unless its pulled and checked. Tome ordering a breeched barrel is a waste of time. Had it been a percussion and shot with corrosive "replica" powder the breech probably would have been rusted in.

Dan

[Elnathan]

If it was not cleaned after shooting pyrodex, even for a few days I would not worry about looking at it. Its no longer usable so just plan on replacing it.
I NEVER remove a liner. I learned this LONG ago. Like 40 years ago. If its put together right there is no need. Seating the liner against a shoulder means there is no need and I sure don't want one with a screw driver slot. Not only is it ugly its not needed. Shouldering the liner drastically reduces the pressure on the part making it even safer. Unfortunately most of the store bought liners don't allow this.
Dan

I wasn't clear enough, I guess. The teenager in question was my younger brother, back in the day. I'm sure he cleaned it, I'm just not sure how well, since I was off at college at the time and not around to supervise him, and it has been sitting around in a closet since then.

I was talking about trying to disassemble a long, counterbored breech in which the liner is inserted through the breech threads to reach the powder chamber, such as in the diagram Blacksmoke posted. I'd have to remove the vent liner to unscrew the breech in that scenario.

[Scota4570]

[/quote]

If you run the tap drill into the plug like the photo above, it will not be supported on one side and will then make an oblong hole.  When the hole is tapped for the liner you will only get a partial thread.  A fine thread like 1/4x32 makes it worse.  This increases the likelihood the liner will be blown out. 

[Mike Brooks]

If you run the tap drill into the plug like the photo above, it will not be supported on one side and will then make an oblong hole.  When the hole is tapped for the liner you will only get a partial thread.  A fine thread like 1/4x32 makes it worse.  This increases the likelihood the liner will be blown out.
[/quote]
No, the vent liner will be supported on all sides by the barrel. The vent liner will stop at the point it enters the bore.

[ron w]

what i'm gathering here is that in most cases, the lock panels of stocks ae actually too far back on the stock, or the barrel channels aren't being opened up far enough back by the builders to position the vent holes where they should be placed on the barrel. I assume these photos are associated with barrels used on pre-carved stocks, anyways.  if the former is trye,...why is it that the pre-carved stock suppliers don't reposition the lock panels and barrel channel ends to accommodate correctly installed vent holes.
   I guess this is one good reason to start from a blank, given the builde understands the proper layout needed.
   maybe there's a need for a precarved stock that has just the butt stock and the beginning of the grip area shaped up, with the lock panel area left as a plank and the forend just left square. it would still save some time and would accommodate getting the lock panel flash hole layout association correct.
   then, a builder could get the barrel/plug/ flash hole in the right position first, before the lock panel is let in.
   I understand that a lot of this is the small details that separates schools of design, but building a correctly functioning gun with it's components in the correct locations with safe construction, is far more important than accomplishing any accuracy in copying some architectural school.

just my opinion,.....does this make sense ?. i would much rather see a suggestion of any particular school be only a suggestion because i built the gun with it's parts in the correct relationship.

[Scota4570]

I was not clear....After you drill through the barrel and hit the plug, you are now making an interrupted cut.  The drill bit will then put a side load on the muzzle side of the hole in the barrel.  It will then continue to cut the hole in the barrel and make it oblong to some degree. 

[ron w]

that is correct, and the reason is because of the location of the lock panel on the stock. …..in order to get the cock to strike the nipple or the frizzen correctly, the lock has to be in a specific location. when that location isn't coordinated with the location of the flash hole in the barrel, parts start getting moved around and putting a divot in the face of the plug is the usual remedy. if the lock panel on the stock is in a better location, the plug can be installed without interrupting it's threads and the liner hole can be drilled without the drill bit hitting the face of the plug,...as it should be done.

[Scota4570]

Pre-carves/ pre-inletted  can be more trouble than they are worth. Sometimes  time I spend scratching my head on how to work around what is there is more than if I just started from a plank.  I would definitely determine if the lock inlet is correct before I cut on it.  Send it back if it is wrong.   Better yet, get the stock with no lock inlet. 

[rich pierce]

Originals were not built from precarved stocks yet most drum percussion guns have the drum going into the front edge of the breech plug. 

I hypothesize that builders back then really wanted ignition to happen at the rear of the  powder charge, and in their experience building and repairing guns, somewhere between 3/8”and 1/2” of threads was just right.

[ron w]

I have read that the builders were somewhat anal about that. still,...it would seem to me that it isn't all that hard to get the flash hole in the right place. a few simple measurements is all it takes one the plug is fit. I see no reason to drill a flash hole anywhere before I know i've got the plug well fit. to me, it's just back-a$$wards to do it any other way and if the lock panels, or lock inlet, or barrel channel isn't right, the stock should go back to the supplier. or be corrected if it can, before it leads to having to grind on the plug face.

[Mike Brooks]

On many kits the barrel has to be moved back 1/4" or more to get the touch hole to line up to what we consider "the correct position". Most newbies don't realize this or some folks are just too lazy to back them up that far.

[Mike Brooks]

UH-OH. Just had another thought about possible disaster. What about vent liners? If they don't shut off against a shoulder like breech plugs do won't gases and crud creep up the threads and rot the threads out until the vent liner blows out? Seems a sure thing, even with the way Hugh and Jerry put there liners in chambered breeches.

 I'm so paranoid now I think I'll give this up and start quilting with my wife. Wait, she uses sharp needles. I could stab my self and bleed to death.

[rich pierce]

No, Mike, it’s different when the hole is threaded from the side. Just kidding!  I think vent liners are just as controversial as breechplugs. I guess it’s the shoulder at the surface of the barrel that is the clincher?

[HelmutKutz]

If I misunderstand, please explain because I am at a loss to understand why there is such argument against engineering based logic.
HK

Helmut, I don't think that more than a handful of guns made, both today and in the past, that meet those standards. Even if the breechplugs meet them, I suspect quite a number of nipples, drums, and vent liners do not have enough threads. I don't believe that there are any components off the shelf that can be made to work - Even Green Mountain barrels only have about 5/8" of threads in them, and the plugs available have about a 1/8" gap at the end between the threads and the tang area, which means that about 1/2" is the max one can get without reworking the barrel breech or making a plug from scratch. For those of us without access to a lathe or the skill do use one, that means sending EVERY barrel out to be reworked. That complicates things a mite.

At the same time, there just doesn't seem to be much evidence that plugs fail, unless they are very poorly installed (nobody is seriously arguing that plugs are OK with just three threads - most plugs, notched or otherwise, seem to have around five or six).  Given that these figures also have weight of several hundred years of trial and error, I can see why some people might be a bit skeptical.

I do appreciate you sharing your knowledge with us, and I'm sure that there are others here as well. I'm certainly not ignoring it.

I still beg for an answer as to why anyone would argue against the proven data when it's not beyond reason to create acceptable designs. Certainly we are limited no longer by available technology tooling, yet there is certainly an underlying insistence in accepting some while rejecting others.

I previously asked why heavy course threads are held up as comparison to modern fine threads when the load bearing and stress factors are completely different and can easily be modeled by FEA (Finite Element Analysis) software or even destructive testing if one wishes to observe the physical specimens.

One said something about the force being less on the plug than the barrel which is not correct as all pressure is equal at any given point and the force applied via flow dynamics in response to rapid pressure or flow changes such as those we observe when combustion is contained within a pressure vessel as being discussed here. Furthermore, the same rapid pressure changes and flow dynamics also affect every other parts of the assembly like vent liners in unique ways.

In design engineering there is no acceptable failure rate, thus we design things with a certain amount of safety factor so as to eliminate the chance of failure under the acceptable use conditions. My greatest concern here are those who make arguments which seemingly assume some acceptable failure rate based solely upon the lack of data. To me this is like attempting to prove a negative.
HK

[Pennsylvania Dutchman]

Helmut, I believe your questions have been answered by real world proof. Barring human error, I.E. stupidity or distraction in loading, 1/2" long breech plugs, regardless of their diameter, are not blowing out, knotched breech plugs are not blowing out, and 12L14 steel barrels are not blowing apart. Zero failure rate is unachievable as long as there is a human element involved, even properly built and maintained steam boilers have blown up due to human error. For azero failure rate we would probably need the 12" long breech plug Mike mentioned along with a 3" wall thickness on a .45 caliber barrel in case someone loaded it improperly.
Mark Poley

[T*O*F]

rot the threads out until the vent liner blows out?

Lest we forget, most vent liners are retained in the barrel by the lock bolster making it near impossible to blow the liner out.  Also, by the time the threads are rotted out, the touch hole is likely so big that it relieves any pressure build up.

[Daryl]

One saving grace for liners - and drums for that matter.  The smaller the threaded diameter, the lower the actual pressure in psi. By thee same token, the larger the diameter, the higher the pressure.

There is more pressure against the threads of a 3/4" breech, than a 3/8" breech.

Crappy machining is why drums blow out, now and then - not enough threads?

[Mike Brooks]

rot the threads out until the vent liner blows out?

Lest we forget, most vent liners are retained in the barrel by the lock bolster making it near impossible to blow the liner out.  Also, by the time the threads are rotted out, the touch hole is likely so big that it relieves any pressure build up.

Read my previous post. I have seen a blown out vent liner. Took the lock, screws and sideplate with it shattering the stock

[jerrywh]

 Fool proof is largely determined by the size of the fool. I have removed breech plugs from original barrels 150 years old that had no seat whatsoever. The barrels were no longer safe to shoot in my opinion but the plugs were as good as when installed and took a great amount of force to remove. The threads were a force fit and rounded. The touch holes or percussion hole went through the plugs with no liners.