Breach plug length

[Chris in Washington]

Just wondering... what do you consider the minimum length for a breach plug?

I've got a .40 cal barrel, 1 inch across the flat at the breach.  5/8th in plug.  The lock that I would like to use is 3/8th from the fence to center of pan.  Can I safely shorten the breach plug to 3/8 to get the fence and the end of the barrel to line up?

Chris

[Don Getz]

Why?  I will admit, 5/8" is too long, 1/2" is plenty, but I would not shorten it up to 3/8".   To go back to my first question, why do you necessarily want the fence to line up with the end of the barrel?    Are you doing it because someone said it
should be done that way?   Look at old rifles, and check out the guns in Rifles of Colonial America, some were done that
way and some were not, this is not a hard and fast rule.   If you are building a shooting gun, I would prefer to place the
touchole in front of the breech plug, without cutting a notch into it, then let the lock fall where it may.    When you look
at old guns, many of the locks were not built like a siler, they had a bigger pan, and more distance back to the fence.
Also, most old guns did not have touchole liners, just a hole in the barrel, this would allow a more rearward placement of
the lock.  On the other hand, I have seen old breech plugs with a notch filed in them to allow a more rearward placement
of the lock.   What I don't like about this is, it's a place that becomes hard to clean and will build up $#@* if you shoot a lot............Don

[Acer Saccharum]

Don, my sediments exactly. Well said. Going throught the jaeger book, Steinschloss Jaegerbuschen, I see the same thing. Some are lined up, some are not. It's definitely not worth sacrificing performance for appearance.

Acer

[Long John]

Chris,

Listen to Uncle Don!  Acer too!

This topic came up about 2 years ago.  I never line up the fence with the rear of the barrel - I locate the lock relative to the barrel at the position for optimum ignition speed (flash hole centered on pan and tangent to pan top surface).  I thought I was screwing-up.  So I went through both volumes of Rifles of Colonial America, tabulating whether the pan fence was lined up with the back of the breech.  Over 60 % were not.  About 25% were and the rest I could not determin it from the pictures.

The easiest to clean breech is one with a flat, polished surface.  Remove the breech plug. Polish the breech plug face down to 600 grit removing as little metal as possible.  Coat it with soot and screw it in to the witness mark.  Now carefully remove the breech plug and look at the soot on the face.  You want to see a circle around the perimeter of the breech plug face where the soot has been taken off.  If you don't see it this means that the face is not up snug against the ledge formed by the bore and the female threaded hole in the breech end of the barrel.  That is an issue that should be remedied before anything else.

Best Regards,

JMC

[Clowdis]

Didn't this thought get started by some of the judging at Dixon's? I think I read that somewhere in an older post. Just curious.
Blair

[T*O*F]

Don, my sediments exactly.

Sediment......the dirt that sinks to the bottom with the other trash.

Both you and Don are machinists and should know how many threads are required to achieve maximum holding strength and that additional threads do not add additional strength.

Strongly opinionated persons become embroiled in these discussions about lock and touch hole locations while ignoriing the above fact.  The plug need only be as long as safely required for strength and that is what determines touch hole location.

Almost all commercially available plugs are made 5/8ths long.  However, required length varies with plug diameter......a 3/4" plug necessarily must be longer than a 9/16" plug.  Yet, everyone generalizes details while ignoring specific details.  How is a newby supposed to come to any conclusions without specifc, rather than general facts?

[Roger Fisher]

Don, my sediments exactly.

Sediment......the dirt that sinks to the bottom with the other trash.

Both you and Don are machinists and should know how many threads are required to achieve maximum holding strength and that additional threads do not add additional strength.

Strongly opinionated persons become embroiled in these discussions about lock and touch hole locations while ignoriing the above fact.  The plug need only be as long as safely required for strength and that is what determines touch hole location.

Almost all commercially available plugs are made 5/8ths long.  However, required length varies with plug diameter......a 3/4" plug necessarily must be longer than a 9/16" plug.  Yet, everyone generalizes details while ignoring specific details.  How is a newby supposed to come to any conclusions without specifc, rather than general facts?

Well Gee!     ALL, the previous posts held forth excellent information in my book!

[Robby]

I agree Roger, excellent information. I'm also in agreement with Old Fox, I would be very interested in seeing what is a safe minimum, from an engineering point of view, for the various size plugs. Not that they would have the last word on the subject, after all engineers have proven that bumblebee's can't fly.

[Pete G.]

I think that 3/8" is sufficeint if everything is perfect (and stays that way), but leaves very little margin for error. I have always wondered if notching the plug, which was/is very common compromised the intergrity of the breachplug seal. Sure seems like it does, but it seems to work just fine.

[Acer Saccharum]

Sediment......the dirt that sinks to the bottom with the other trash.

Both you and Don are machinists and should know how many threads are required to achieve maximum holding strength and that additional threads do not add additional strength.

Ouch, ooh, ouch, Dave. That felt like a slap. But maybe I'm just reading too much into it, maybe a little too sensitive today.

So I recommend a certain plug length, and Joe Newbie goes by what I say, and blows his head off. No, sir, there are too many variables that come into play for me to give you a blanket statement for breechplug length.

Acer

[D. Taylor Sapergia]

Here's what I found when I removed the plugs from my 14 gauge Staudenmayer pair of flint barrels.  The plugs fit perfectly, were tight right to the ends of their threads, and were 7/8" dia. x 3/8" long.

With a perfect installation, I'd be comfortable with a 7/16" long plug in a .40 cal rifle.  But don't get hung up on the breech face/lock fence thing. 

[Tim Crosby]

Yet, everyone generalizes details while ignoring specific details.  How is a newby supposed to come to any conclusions without specifc, rather than general facts?

 5/8s is to long but 1/2" is plenty but not to go to 3/8s sounds pretty specific to me.

Tim C.

[T*O*F]

Ouch, ooh, ouch, Dave. That felt like a slap. But maybe I'm just reading too much into it, maybe a little too sensitive today.

Tom, perhaps we should call you "Acher" today.  Not a slap, just a gentle nudge at your Freudian slip cause I know you are an ejikated speller.  There is no "nt" in sediment.

So I recommend a certain plug length, and Joe Newbie goes by what I say, and blows his head off. No, sir, there are too many variables that come into play for me to give you a blanket statement for breechplug length.

It's not your recommendation that is the problem, it's Joe Newbie's ignorance of things mechanical; ie, the variables you mention

If you look at Taylor's picture, those plugs have 5 threads.  Three FULL threads are required for maximum holding strength.  The other two are partial threads. 

Now we know that tap drills are chosen to give 75% contact.  Also that there are different taps giving tighter or looser toleranances and dies that are screw adjustable to match the screw to the threaded hole.  Joe doesn't know this.  He's bought a cheap set of Chinese tools and runs them down the hole to clean it up and also over his commercial breechplug, shaving off excess metal in the process.  In doing so, he has reduced the thread contact to 50% or less.  The plug wobbles in the threads, but he doesn't know that this isn't normal.  However, he has reduced that plug below its safe operating parameters.  Then he overcranks the the plug and stretches those threads in the process further reducing their safety margin. 

He brags to his buddies about his gunsmithing prowess.  Later at the range, he notices every time he shoots the gun, smoke comes from the back of the barrel.  He is puzzled why and takes it to a professional who fixes it properly.  His forehead remains unmarked and he retains his eyesight.  He's one of the lucky ones.

The greater sin of the whole story is that those who know better do not intercede when poor advice is given and the sediment remains in suspension instead of sinking to the bottom where it belongs.  (Part 2 to follow later)

[Stophel]

I'd go 7/16".  The pan fence will be slightly ahead of the breech end of the barrel, but so what?

 

[jerrywh]

Shotgun pressures are a lot lower than rifle pressures. Iexcept for the cleaning problem, I don't mind cutting a notch in the beech plug. The plug has to turn sideways before it can come out.  My theory is the plug has to be as strong as the barrel at the breech. 3/8" on a rifle gives me some worry. 1/2" doesn't worry me at all if it is a good fit and good steel. I don't make plugs with a 75% fit like the standard for bolts and nuts in this country. I cut the plug oversized on the  lathe. I like a 90% fit.
   Has anybody on this forum ever seen a breech plug blow? in 55 years I never have.

[Don Getz]

As a general rule of thumb, we always used 1/2" deep plugs, and , fortunately, never had a problem.  I am not an engineer, a trained tool and die maker, or even a trained machinist.   I had on the job training.   When we first bought the
Paris barrel business, they had us making coarse thread plugs.....3/4-10, 5/8-11 threads, similar to the old barrels.  I don't
think there are many people on this forum that have ever done these, but, it is virtually impossible to make them
waterproof.  When you look at these, the threads are big, and apparently will stand the pressures present in a muzzleloading gun.   One other thing aboout these coarse threads....we chased all of the breech plugs on a lathe....very
time consuming.    After several months in the barrel business, we went to the fine threads....5/89-18, 3/4-16, etc., mainly due to pressure from certain individuals whom I choose not to mention at this time.   We had an old turret lathe
which we brought along from the Paris shop, but never used it.  We bought a threading head and now use it to thread
breech plug blanks in the fine threads.....sure speeded up the operation.   We have always stuck with the 1/2" deep threads, regardless of the size of the plug....5/8", 3/4" or 7/8", unless we were fitting a patent breech, which normally
came with a longer threaded shank.   We have done a lot of proofing of barrels and never had a plug blown out of the barrel.  I think you could possibly do shorter threaded breech plugs, assuming it was done with extreme care.  When you
are working in a somewhat production mode, you cannot take the time to "fit" every plug.....using layout fluid to se where
you are hitting, correct it and try again, etc......talk about a way to lose money.   John Bivins used to ask what sore of
"crunch" factor we used.  One cannot describe this, but, you make a plug, threaded, and screw in the round blank, using
a 14" pipe wrench and you actually crunch the plug against the bottom of the threaded breech...done.   That is what we
called fitting.  I'm sure all of the other companies do the same thing.   So there you have it..............Don

[Acer Saccharum]

Not all companies 'fit' the plugs, Don. I always check the fit to get a tight seal/fit on plug face to bore. Takes at least an hour to get fitted up right. Sometimes you have to take material off the back of the barrel to screw the plug in so it hits the bore.

I hate having a fouling gap at the bottom of the bore. It's not unsafe, it's just miserable to clean.

Acer

[Daryl]

Jerry hit the nail on the head - pressure!  A .40 or .45 develops more pressure with  65gr. of 3F than does a 14 bore with 200gr. 2F.  The .40 and .45 rifles require more good threads (deeper) given identical wall thickness, than the 14 bore does as their pressure is higher - much higher.
An outfit back East says they've been trying to blow up a Bess I sent back that had 3 threads engaging, with an average of .0216" engagement. Actually, one thread engaged .028", then .022" and .015".  I won't call the last one, the 4th, with .002" engagement as being an 'working'  thread.  Now, how much is required to be safe?  That Bess, a .75, according to them, has digested many? 'proof' loads of 200gr. 3F and a tightly patched round ball without so much as any leakage. That part is interesting as only the 'flange' attached to the tang, which pressed against the end of the barrel, kept all that gas inside. The threads filled in only 5 shots. Perhaps those filled threads increased the 'strength' of the threading job - or at least, stopped the gas leakage.

Just how much is necessary for strength?

[Acer Saccharum]

What is necessary for strength?

What kind of thread interference do you have? if there is a sloppy fit, then I would not consider any length of plug to be safe.

Using a substandard grade of steel? Iron? Perhaps the threads would not be as strong as a nice uniform low carbon steel.

It is very difficult to tell someone a simple answer, especially when you don't know how much the other party knows about threading, fitting, what kind of equipment they have on hand. What tolerance tap do they have? Does their die cut threads to a good fit in the breech? Did their tap run crooked, and then they run it in again to make the threading of the breech straighter?

I would be completely happy with 3/8 engagement, IF I knew the quality of the threads in both the breech and on the plug, and the caliber of the gun. I want almost all those threads working for me.

[flehto]

The force generated by the ignited powder against the plug face is area times the pressure of the bore. If the plug face seals against the shoulder, only the pressure of the bore  registers against the threaded plug. so any number of combinations of bore diameters and generated pressures are possible and the force against the plug threads can be calculated if the pressure is known. Possibly the longer plug lengths are a supposed "safety factor" in case of liability suits seeing good engineering practice dictates threaded  length equal to thread diameter. Until definitive test rsults are known, all conjecture on this subject is a "guessing game".....Fred

[jerrywh]

 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.

[flehto]

The pressure in a bbl is testing the tensile strength of the bbl steel. The threaded plug is resisting failure due to the shear strength of the steel in the threads and a wedging open of the threaded portion of the bbl  which would reflect  the tensile strength of the bbl steel. If a pressure cap is retained by screws, both tensile strength of the screw and thread shear strength would be tested and the weaker of the two would  cause a leak or blown cap. Because of the expansion of the bbl due to excessive pressure, the bbl/plug threads would have less engagement and therefore less shear strength of the plug or bbl threads and wedging would occur more easily.  Most if not all CFs  have the bbl screwing into the receiver so  excessive pressure increases thread engagement.... the opposite of most MLing bbls and plugs.... Many factors determine how much  force is req'd to blow out a plug and steel type of both the bbl and plug and  thread engagement are 2 of the main factors. I haven't seen any blown or bulged bbls but in either case, was the plug still engaged? If so, the bbl was the weak link and not the threaded plug. I agree w/ Jerry on what he stated....Fred

[J Shingler]

I have only seen one blown barrel in my time. It was the result of some halfwit using smokeless powder. If I am remembering correctly the barrel was split in three pf for about a foot in length starting at the breach. Looked like a tripod. Kind of like those cigars in the cartoons. Anyway the threads still looked good with no indication of stripping. The barrel sure blew though so I would agree with Jerry as well.
Jeff

[Dphariss]

Just wondering... what do you consider the minimum length for a breach plug?

I've got a .40 cal barrel, 1 inch across the flat at the breach.  5/8th in plug.  The lock that I would like to use is 3/8th from the fence to center of pan.  Can I safely shorten the breach plug to 3/8 to get the fence and the end of the barrel to line up?

Chris

How small is the lock? 3/8 from the center of the pan to the back of the fence is pretty short.

I like to have the fence at the end of the barrel, it looks better, but I also like 1/2" of threads. Also note that many store bought plugs have a significant rebate of the threads at the tang end so a thread or so is lost.

In your case I would either rethink the lock or set it ahead 1/8" or so.

Dan

[Chris in Washington]

OK....  so it looks like I really open up a can of worms 
The main reason for doing it was because I though that I looked cleaner having the fence and barrel line up.  The lock that I'm using the Chambers "Dale Johnson" lock.    I'll make sure everthing right and tight.  Looks like I might go 7/16 or so. 

Thanks for all the input,
Chris