Breech Plug

[AeroE]

Threads only have so much load that they can impart or resist. If the threads strength is used up in tightening the plug then what is left to take the pressure when the gun is fired?

I have been percolating on this for years. I wonder how much the threads are actually accomplishing. It is not as if this were a pipe bomb where the threads were containing the full force of a powder charge without string support. Rather, the breach is butted up to the stock -- in many cases bedded to perfect the support.

I would be interested in comments from you all on this.

CS

The breech plug constrains the pressure of the powder inside the barrel. The stock reacts the recoil of the bullet leaving the barrel. Two different mechanical systems.

Think about this - if there was no breech plug, would you expect the bullet to travel down the barrel after the powder charge explodes? I hope the answer is, "No, obviously the powder will blow out the aft end of the barrel and the bullet will probably not move at all."

 

[crockett]

Man, am I ever glad I asked that question, thanks a bunch. Not to bad mouth anybody but the outfit I was considering is TOW. How well do they install breech plugs? I don't mind doing the work, would I be better off doing it my self- I'm pretty skilled in such work.

 

[CrackStock]

if there was no breech plug, would you expect the bullet to travel down the barrel after the powder charge explodes? I hope the answer is, "No, obviously the powder will blow out the aft end of the barrel and the bullet will probably not move at all."

I see the 2 functions, but performed by different parts of the gun than you seem to see.

One (the threading) contains pressure leakage and the other (gunstock) absorbs the recoil and retains the plug in the tube.

CS

 

[AeroE]

if there was no breech plug, would you expect the bullet to travel down the barrel after the powder charge explodes? I hope the answer is, "No, obviously the powder will blow out the aft end of the barrel and the bullet will probably not move at all."

I see the 2 functions, but performed by different parts of the gun than you seem to see.

One (the threading) contains pressure leakage and the other (gunstock) absorbs the recoil and retains the plug in the tube.

CS

The threads are the mechanical feature that keeps the breech plug in the barrel; the stock can't possibly perform that function. If a smooth breech plug were pressed into the barrel, it would pop out like a cork when the gun is fired, and although you could probably get enough interference fit to prevent that from happening, the plug installation would not be reliable or trustworthy.

Consider this - the cross section area of a .50 caliber bore is roughly 0.196 square inches. A peak pressure of 20000 psi acting on the end of a breech plug of that area is 0.196(20000) = 3926 pounds. I'm fairly certain that I don't have a wood stock that will react that force.

Let's suppose you laid a barrel with a threaded breech plug on the ground, loaded and ready to fire, only this time you will ignite the charge with a cannon fuse - we would expect the barrel to discharge and probably scoot around on the ground from the recoil, and the breech plug to remain installed in the barrel, held in there by its threads.

Clamp a nut up in your vise, screw and bolt in, and pull on the bolt - the threads are holding the bolt and nut together.

A rifle stock is just a convenient handle that also serves to hold all the mechanical parts together in the correct relationship so they will function, but plugging the barrel ain't one of them.

 

[crockett]

Let me re-state how I see it. The threads of the barrel/plug each take up some of the pressure. For the plug to fail the threads would have to shear off and I believe those in the know call this shear strength or something similar. If you screw in the plug too tight you are already putting some stress on the treads. This is a really good point because I would have thought you should screw in the breech plug as tight as possible. I have seen various wrenches used for breech plugs and the size of the wrench gives the impression you really have to "crank it in". I am now getting the impression you should be able to screw the plug in without much effort. Is that correct? Should some type of anti-seize compound always be used on the threads and what kind of compound? It is now my impression that the $20.00 you pay to have this done is for the proper fitting, not because is it difficult to screw in the plug- requiring some type of monterous wrench.
Any further advice most welcomed.

 

[WRussell]

Another mechanical issue that's relevant here is that you can't expect a bolt to stay in place unless it is stretched just a bit. You can screw in a breech plug finger tight and it will not blow out, but you can't expect it to stay tight.

The trick is to put just a little bit of stretch on it (that's what torque wrenches are for) so you haven't used up much of it's strength but you can rely on it staying in place.

On a muzzleloader, the breech plug can't loosen very much because of the tang screw, so I expect there are guns out there that have served just fine for years with finger tight breech plugs, but it surely isn't a good situation.

Another way to look at the picture of the wood at the breech holding the plug in place is this: say the plug is blowing out, but the wood behind it is made of super-wood, so it won't give. What's the barrel going to do? Yup, the barrel is going to try to blow right off the breech plug. What's holding the barrel in place? Just the flimsy fore-end.

Nope, the wood has nothing to do with keeping the plug in there. If the threads don't do their job there will be damage done.

 

[CrackStock]

You have an interesting theory there about the barrel skittering about on the ground, but I still see that the rearward pressure is absorbed by the wood at the breach. Some may be dampened by the weight of the barrel, but it does impact the stock. Otherwise, why does everyone carefully explain that the barrel and breach plug nest tightly at the breach? It is to support the breach plug.

Show me an engineering model that demonstrates that the wood does not support the breach plug.

CS

 

[TANSTAAFL]

Links might help understand how a fastner takes a load, almost all of it is applied to the first three threads.

(may take a bit for page) http://72.14.203.104/search?q=cache:MxLB55Zcf7YJ:www.gizmology.net/nutsbolts.htm+first+three+fastener+threads+take+the+load&hl=en&gl=us&ct=clnk&cd=1
[url] http://www.boltscience.com/pages/strength.htm[/url]

 

[Celt]

Speaking as one who's only built three or four rifles, but who has a machining and welding background.....
If one were the slightest bit worried the structural integrity of their breechplug, what would be the pros and cons of cross-drilling through the barrel and plug and pinning the whole thing, just for insurance? A light press with a 1/8" hardened pin, flush with the side barrel flats, perhaps?

 

[AeroE]

You have an interesting theory there about the barrel skittering about on the ground, but I still see that the rearward pressure is absorbed by the wood at the breach. Some may be dampened by the weight of the barrel, but it does impact the stock. Otherwise, why does everyone carefully explain that the barrel and breach plug nest tightly at the breach? It is to support the breach plug.

Show me an engineering model that demonstrates that the wood does not support the breach plug.

CS

Crackstock - please see your private topics.

Please answer for yourself what happens to the breech plug in a barrel that is not installed in a stock when the charge is ignited.

The only other suggestion I can think of if that fails is that you need to unbreech one of your guns, reinstall the barrel in the stock, and shoot a ball down range. Please fire the gun with a string to the trigger if you choose this experiment.

"Skittering" is an overstatement that means "move during recoil". It's not a theory, it's fact; a fact that is illustrated every time you fire one of your guns and it recoils, unless you are shooting feathers with light charges of powder - who knows, I've heard of stranger things.

 

[TANSTAAFL]

If properly breeched, there is naught to fear with the standard threaded procedure.

 

[CrackStock]

Cross drilling is overkill. Most commercial threads are well capable of holding the plugs in place.

We are discussing how much support the breach portion of the stock could support a weak plug.

My curiousity stems from a much earlier discussion of an inexpensive gun which some people claimed had weak threads.

We also had a recent discussion regarding improperly installed plugs in which the flat of the plug did not meet with the cut ends of the lands in the rifling. This leaves an area which never gets properly cleaned or protected. Some say that this does not matter and others take pains to see that plugs are carefully mated.

I just wanted to hear some descriptions of the effects of poorly threaded plugs and tossed out devil's advocate remarks for that purpose.

It is clear that the threads have strength in order to prevent leakage, but what I was getting at was how much the stock could retain with a weakly threaded plug.

CS

 

[crockett]

I'm no expert so excuse my ignorance but aren't we talking about different kinds of forces being exerted?
The threads on the breech plug have to do with holding the breech plug in the barrel, if the pressure is so great as to strip the plug, an inch or two of hardwood in the stock isn't going to hold the plug. You could probably hit the stock in that area with a hammer and split off the wood. If old barrels were made of soft iron so the bore wouldn't shoot slick, then I would guess today's steel barrels and plugs are more than strong enough- unless torque, etc comes into play.
The other issue is how the recoil of the entire gun operates, I think the reason for the tight fit is so everything moves in unison, otherwise a loose fitting barrel could bounce around and damage the stock.
Those are my thoughts, willing to listen to others.