What's the minimum safe breach plug ?

[ron w]

acknowledging the 1/2 minimum thread engagement length school of thought,....as long as the short side of the plug leaves at least a 1/2 inch of threads,.....the plug is safe......given class 3a&b threads are used.
    it would be interesting to know how the plug threads of the original guns compare to modern classification.

[Daryl]

This is what I have(this is a very close to what is in my rifle)

So reading the above 2 pages, it seems that plug would be safe Without the notch,,,, but is Not Safe with the notch?
If that's the case, then the unmolested plug with 7 full threads is only marginally safe to begin with?
Really? If that's the case, these muzzle loaders should be blowing up practically every day! Not to mention the old original ones with nothing more than 3 course threads, with a notch filed in them..... that have somehow managed to exist this long....

I don't see anything wrong with this plug, as long as the threads actually prevent powder gasses from escaping past the breech, into the stock. Looks fine to me- I had guessed at 8 threads.

[ron w]

there's not much taken off that plug. that said the notch does eliminate the seal with the inside shoulder in the barrel. part of what constitutes a well built rifle is arranging lock, flash hole and barrel locations so that this doesn't need to be done. I realize it sometimes is impossible to avoid when using a pre carved stock.

[Mike Brooks]

I don't want to give anybody a heart attack, but I would wrap some Teflon tape around that rascal and screw it in the barrel and go on with life with out a care in the world. I would rather it wasn't notched but it sure as $#*! isn't going to blow out and the Teflon tape will plug any leaks…. I doubt it will leak the way it is now.

[Long John]

Recurve,

I would expect the breech plug shown in your photo to safely hand any reasonable, properly loaded blackpowder charge.  I don't make breech plugs that way for a different reason.

Straight threads, which is what you have on that breech plug, do NOT seal - there is a small amount of free space between the threads that allows the threaded male part to be turned into the female part.  The sealing comes from the flat surface of the breech plug face being pressed up against the ledge in the breech formed by the larger diameter breech plug hole meeting the smaller diameter bore.  The threads function as an inclined plane to force a tight fit between those two flat surfaces and achieving a seal.  When you notch the face the sealing effect is lost and gas can flow backwards along and between the threads.  The gas includes the vapor of sulfuric acid which is corrosive.  It is POSSIBLE, not guaranteed just possible, that this will lead to corrosion over time.  This is not desirable.  Hence, Mike would use a sealing means to seal up the space between the threads to prevent this corrosion mechanism.  I would use Locktite 242 rather than Teflon tape but that's just what I prefer.

Best Regards,

JMC
John Cholin

[ron w]

it is possible when cuttingthreads with a lathe, to make a very slight interference fit between inside and outside threads, so that the plug turns in under just a bit of tension all the way in.  threads of this type will help seal the breech. high power gunsmiths (especially target gunsmiths) use this slight interference thread design when barreling modern actions to ensure no movement between barrel and action during the shot. it is actually tighter fitting than class 3A&B threads and requires ant-seize compound to be used or the threads will surely gall..

[JTR]

    it would be interesting to know how the plug threads of the original guns compare to modern classification.

I can shed a little light here as I've had a number of originals apart over the years. Basically, when screwing them in, you can screw them in by hand, and you'll only need a wrench for the last 1/4 to 1/2 of a turn. When unscrewing an original breech plug, you'll need a wrench to pop it loose, turn it maybe a half turn with a wrench, then unscrew it by hand. Most of these threads look more like a course thread modern pipe thread.
Most all of the guns I worked on were flint era and early percussion originals. As the guns get closer to the 1850's/60', the threaded fit of all the fasteners tighten up considerably.

That's not to say that there aren't exceptions to what I've said above, because I'm sure there are. That's not to say that some original gun didn't blow a breech plug at some point, but I've never seen an original gun that showed any signs of that, either as broken pieces or as repaired.

Just my 2 cent experience,
John

[shifty]

      Here is another picture of this style breeck plug taken from page 66 of the first edition of the Lyman Muzzleloaders Handbook.

[HelmutKutz]

Remember, the first thread is to be discounted as non-load bearing, thus a minimum of nine full threads are required to achieve the minimum seven full threads of load bearing surface. Adding minimum necessary 125% safety factor increases minimum to 11.25 threads or 0.625" thread length.

[HelmutKutz]

In the section on "Fasteners" you will find your answer to your question.

Fasteners are not calculated in the same manner as threaded pressure vessel fittings.

[HelmutKutz]

This is what I have(this is a very close to what is in my rifle)

So reading the above 2 pages, it seems that plug would be safe Without the notch,,,, but is Not Safe with the notch?
If that's the case, then the unmolested plug with 7 full threads is only marginally safe to begin with?
Really? If that's the case, these muzzle loaders should be blowing up practically every day! Not to mention the old original ones with nothing more than 3 course threads, with a notch filed in them..... that have somehow managed to exist this long....

Bare electrical wires are safe as well until someone touches them or something causes them to contact that which they should not. Steam boilers on locomotives were quite well designed though not properly inspected nor protected for corrosion. Just because one was fired a thousand times does not mean it will not explode on the 1001 firing. Safety factors have been developed mostly by learning from failures. As I responded to the other post, seven load-bearing threads are minimum whereby nine are required to achieve the seven load-bearing and on an 5/8"-18 plug an addition 2.25 threads are required to meet the absolute minimum safety factor, thus 11.25 threads total. Another 125% minimum must be added for any interruption in threads such as the notch or drilled hole as shown by another person.

[Daryl]

Helmut - are you referring to this style breech when referring to a drilled breech?



let my country awake

[Scota4570]

I have been hesitant to get back into this one.  Here goes....

I would never own a gun so breached.  I would never do it that way, it is as easy to do it right as to do it wrong after-all.   I would never buy a gun so breached.  I check plugs before I use a barrel.  IF somebody sold me such a gun I would demand a refund.  Under no circumstance would I shoot it. 

I am opinionated.  That is  based on 50 years around guns and 30 years as a forensic scientist specializing in firearms matters.

Catching a plug in the face is pretty serious, you will probably die.   There is no good reason to do such sloppy work.  If it ,"came out wrong" then start over and do it right. 

Antique guns have no sway with me.  We know better now, so do it right.   

[HelmutKutz]

Helmut - are you referring to this style breech when referring to a drilled breech?



let my country awake

Shown in the picture are eight load bearing threads which itself is insufficient for a 5/8"-18 plug as explained previously, such requires a minimum of 11.25 threads. Perhaps drawing is not to scale but it appears to be fairly accurate considering the 0.440" liner center measure.

The area of the interruption, this case being 0.3125" diameter touch hole liner, is no longer load bearing and must be accounted for at 125% compensation factor.

[JTR]

Looks like we're working up to Mikes' 12 inch long breech plug again 

[StevenV]

some pics of original breech plugs for your viewing pleasure          StevenV

[StevenV]

breech plug for current build          StevenV

[rich pierce]

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?

A good many of the originals pictured here have been shot a lot. Some were likely in service for a hundred years. Having disassembled a decent number of drum and nipple percussion rifles, the majority have the drum set into the front of the breechplug with relief of part of the threaded portion. The drum must be removed before the breechplug.

[Mike Brooks]

I'll tell y'all (practicing my southern speak for the Knoxville show ) a little story of a gun that came across my bench 20 or more years ago. It was a blowed up TVLLE made by Allegany (I think). The owner was drunk and felt threatened by hostiles on snowmobiles whilst camping in the dead of winter in a Tee Pee. So he grabs his gun and horn and dumps who knows how much powder down the barrel straight from the horn (like in the movies ) and plops a ball on top. Staggers outside and takes a pot shot at said scary hostiles. TVLLE promptly comes apart. Vent liner exits barrel taking lock and lock screws plus sideplate  plus much wood with it. TVLLE is now laying on the ground in many pieces and Mr. Drunk says " Golly, my gun broke!" Or something to that effect.

So anyway, the TVLLE parts end up on my bench sometime later to get a quote on a possible restock. "Not likely" says I. Main reason is I don't want this guy owning one of my guns an me facing a law suit some day because of a repeat situation. Secondly, the barrel appeared to be completely unharmed except it didn't have a vent liner in it anymore. But, I wondered about the stress of being blowed up and it's possible effects on the barrels future.

 So, a catastophic failure and the breech plug held in there on a 20 bore with a massive overload. I don't recall if it was a 1/2" or 5/8" long plug, but I'm not sure if that makes a difference in this case. Oh, 12L14 barrel too.


Next scenario: This happened to me. 1" X .50 Douglas barrel. 80gr 3f. Loaded powder,patch ball, rammed home. Then was distracted and again loaded powder and short started the ball and was distracted again before I could ram it home, I was then called to the line as it as my turn to shoot. I pulled the trigger. Catastophic failure. Gun shatters . Top flat disapears for ever. 5/8" long Breech plug is still in the stock.

Next scenario: A customer loans his neighbor his high end flint pistol I built for him several years before. It is  to be used as a New Years Eve party noise maker . Said pistol is Sterling mounted and has yards or silver wire in it. I wasn't there so I can't say what exactly happened but I suspect the booms weren't loud enough so increasing amounts of powder were drunkenly added trying to achieve the ultimate boom. High end wired pistol stock finally shatters. Barrel and plug stay intact. (.50 smoothbore 1/2" long plug, 12L14) Everyone is highly disappointed. Pistol ends up on my bench for a "please fix now" . This was 15 or 20 years ago. The shattered pistol stays in a drawer, now and forever. You blow up a gun of mine like that, you don't get it back.


My conclusion: The barrel or vent liner will fail before the plug blows out. Of course it's nice if everyone loads their gun properly so we don't have to worry about how it may or may not come apart. I know I pay alot more attention while I load and always run my rod down the bore when I get to the line to shoot just to make sure I have loaded properly even though I know I did.

[HelmutKutz]

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?

A good many of the originals pictured here have been shot a lot. Some were likely in service for a hundred years. Having disassembled a decent number of drum and nipple percussion rifles, the majority have the drum set into the front of the breechplug with relief of part of the threaded portion. The drum must be removed before the breechplug.

Again I say is the locomotive steam boiler that explodes on the 1001 firing still considered "safe" because it didn't explode on the 1000 previous firings?

Safety measures are derived primarily from examining previous failures and the engineering minimums for pressure vessels are different not. But for sake of argument what point is there in showing only those which have not failed when there is no means to show those which have failed? If there were basis in engineering to support fewer course threads, why then have we standards for multiple finer threads in all modern applications? How does one argue against the NPT standardization whereby the minimum number of load-bearing threads ranges from 7.13 to 12.1 respectively for nominal sizes 1/8"-6" and further wonder is why 8tpi on a 6" diameter pipe and not say course thread of 3tpi based on argument here?

The question asked is, "What's the minimum safe breach plug?" to which the answers are given relative to the accepted engineering data which has been derived over centuries of application. I just wish you to provide basis for the argument of few course thread originals when you are not using that in modern application. Sorry but this is like arguing for modern steam boilers to be assembled with wrought iron rivets rather than modern arc welding because we see that some have managed to survive while ignoring all those that have exploded. If I misunderstand, please explain because I am at a loss to understand why there is such argument against engineering based logic.
HK

[Daryl]

TKS Helmut, for answering my question.

[Elnathan]

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.

[JTR]

Given that there must be at least hundreds of thousands of shots made each year with muzzle loaders, let's see some pictures of blown breech plugs.

One's blown with smokeless powder or dynamite don't count!

[Elnathan]

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.

[Dphariss]

Here is the breech plug in my rifle green mtn barrel  36 inch length (internal 351/2) but the touch hole is 1/4 x28 and is located just over 3/8 inch from the breech end of barrel.   the plug is 1/2 and is filed to  3/8 on the touch hole side .  The rifle has a date 1-91

  1 is this safe ?
  2 the builder has marked max load 100 3fff ?
  3  can/should this be replace(and at what cost)?

Leaks gas and fouling into the threads. I never do this.

Dan