Barrel blank

[russellshaffer]

I have a 12 gauge barrel blank that I want to use for a matchlock. It is advertised as being 4140 steel. It is not tapered at all and the wall thickness is .165. So it isn't real thin but it isn't thick either. I don't know what this barrel was intended for because if it was chambered for a shell it would be really thin at the breech end. I am not planning on heavy rifle loads in this so do you guys think it will be safe? I'm thinking of around 70 grains of FF.

 

[kansas_volunteer]

You can easily find online the formula for calculating the bursting strength of a tube. Lyman's manual can tell you pressures. Crunching the numbers, only a few of them, will quickly give you an objective answer.

 

[russellshaffer]

You can easily find online the formula for calculating the bursting strength of a tube. Lyman's manual can tell you pressures. Crunching the numbers, only a few of them, will quickly give you an objective answer.

I don't know how good my research is but I get at least 30,000 PSI yield strength. If this means what I think, it is more than strong enough even for a smokeless factory load. So it should be just fine for a light black powder load.

 

[crankshaft]

search...there have been threads in the past about using chambered bbls for ML guns. the chamber could be a problem with patched RBs.

 

[russellshaffer]

I should have mentioned that my barrel is unchambered.

 

[BrownBear]

I wonder about threads for the breech plug. Not familiar with matchlocks, but if the plug is threaded in, things are going to get real thin back there.

 

[brayhaven]

If it’s 4140, I wouldn’t worry about it. Plenty of margin for error. As for threads, that’s nearly 3/16” wall thickness . I recall the TC oddball breechplug threads of 11/16x20. They used that on everything including 13/16 barrels. Which left 1/16” a side. Properly fitted, you shouldn’t have any gas pressure in the threads anyway. GM used 1137 steel that wasn’t nearly as strong as 4140. Also recall Gen Julian Hatcher trying to blow up a Springfield 30/06 barrel (aisi4140). He kept turning the chamber area down and shooting it. Finally had it down to 1/16” over the chamber. Still it wouldn’t let go with std 55,000psi loads. He had to put high pressure proof loads into it and IIRC, had it over70K before it finally went. You’d be lucky to get 10k in that barrel.

 

[Zonie]

I don't know how good my research is but I get at least 30,000 PSI yield strength. If this means what I think, it is more than strong enough even for a smokeless factory load. So it should be just fine for a light black powder load.

When you see values for tensile strength and yield strength, you are looking at a value that describes the strength of the material per square inch of cross-sectional size.

A piece of material like a square bar with a 1" X 1" size, pulled on lengthwise would have those values.

Likewise a piece of square bar that is 1/2" X 1/2" has a cross-sectional size of .5 X .5 = .25 square inches so your material with 30,000 psi yield strength will only be able to resist a pulling force of .25 X 30,000 psi = 7,500 pounds before it passes its yield strength and it won't return to its former length. (That is what the yield strength means.)

There is a lot more going into the strength of a tube than this but if you have a tube with a 7/8" (.875) outside diameter and an inside diameter of .600" it will have a cross-sectional area of .6013 Sq Inches minus .3421 Sq Inches = .2592 Sq Inches cross-sectional area of material. If it was made using your 30,000 psi material it would withstand a axial force of 7,776 pounds before it yielded.

No offense but this is one of the dangers of people looking at engineering information and not understanding exactly how it works and then jumping to the wrong conclusion.

 

[russellshaffer]

Thanks Zonie. Certainly no offense since I really do not understand how it works. Putting my actual barrel measurements into your example I get just over 11,000 pounds. I had to look up axial force to try to understand your example. Now is there a simple way to calculate whatever the force is that would cause the barrel to bulge or burst? My math skills are worse than rusty. My only real concern is that the barrel be stong enough for safety.

 

[brayhaven]

Thanks Zonie. Certainly no offense since I really do not understand how it works. Putting my actual barrel measurements into your example I get just over 11,000 pounds. I had to look up axial force to try to understand your example. Now is there a simple way to calculate whatever the force is that would cause the barrel to bulge or burst? My math skills are worse than rusty. My only real concern is that the barrel be stong enough for safety.

If you applied engineering type tensile strength numbers, many guns would explode every shot. Look at outer cylinder wall thickness on 44 mag revolvers. I always look at what the manufacturers do successfully. Look at barrel thickness in guns, especially Europeam guns that were required to be proof tested. You’ll see that your wall thickness is actually overkill compared to many of those. And they’re nitro proofed. I’d go ahead and build your gun. If you have reservations, proof test it yourself with a very heavy charge @50% over what you plan to normally use. (Fire with a long string with gun secured) Measure the barrel at several points from breech to 12-16” ahead.. before and after “proofing”. You could proof just the barrel also, using jet-x type fuse. 4140 is a very strong, forgiving steel.

 

[Scota4570]

There is a bigger problem.

12 ga Colerain is .720". So 12 ga is 0.720"? A 3/4" x 16TPI plug has a major thread diameter of .750" and a minor diameter of .671". The threads will not work, no shoulder for the plug and insufficient thread engagement.

It sounds like someone is selling mechanical tubing as if it was a gun barrel based on the "not tapered" part of the description.

 

[russellshaffer]

My breech plug will be 7/8. I have to open the barrel a little to thread it. I doesn't look like a DOM tube - the inside is polished and the outside has been turned full length on a lathe. I have no way to verify if it is in fact 4140 steel as advertised. I just have to trust that the seller's information was correct and they have to take the word of whoever sold the item to them. I'll let you know if it fails when I proof it. In fact, I think I will video the test for better or worse. Thanks for the warning about an undersized breech plug.

 

[Zonie]

brahaven: You cannot look at some gun part and draw the conclusions you have made just by using your eyeball.
To rationalize that the cylinder wall thickness is XXX thick so a gun barrel can be XXX and be safe is a dangerous thing to do.
You have no idea what material was used to make the cylinder or what heat treatment was used on it so you have no idea what the strength of the material is. Some of the better steels can be over 250,000 psi strength. Without the proper tempering, even that strength can be dangerous because although it is strong it also may be prone to shattering when it is subjected to the pressure spike created when a gun fires.

Speaking of pressure spikes, folks might find formulas on the web that will give the safe pressures for a tube but these pressures are for static pressures or pressures that increase slowly.
The almost instantaneous pressures created when a barrel is shot can easily destroy a tube that is safe at the same static pressure. This is because shock stresses are quite different from normal stresses and the forces and stresses created by the shock load can easily overcome the strength of a material.
That is why often the expected pressure that will be seen must be multiplied by a factor of at least 3 and 4 is recommended if the value is to be used to calculate the thickness of a gun barrel.

 

[brayhaven]

brahaven: You cannot look at some gun part and draw the conclusions you have made just by using your eyeball.
To rationalize that the cylinder wall thickness is XXX thick so a gun barrel can be XXX and be safe is a dangerous thing to do.
You have no idea what material was used to make the cylinder or what heat treatment was used on it so you have no idea what the strength of the material is. Some of the better steels can be over 250,000 psi strength. Without the proper tempering, even that strength can be dangerous because although it is strong it also may be prone to shattering when it is subjected to the pressure spike created when a gun fires.

Speaking of pressure spikes, folks might find formulas on the web that will give the safe pressures for a tube but these pressures are for static pressures or pressures that increase slowly.
The almost instantaneous pressures created when a barrel is shot can easily destroy a tube that is safe at the same static pressure. This is because shock stresses are quite different from normal stresses and the forces and stresses created by the shock load can easily overcome the strength of a material.
That is why often the expected pressure that will be seen must be multiplied by a factor of at least 3 and 4 is recommended if the value is to be used to calculate the thickness of a gun barrel.

Not really. Only a few gun steels are used these days. Mostly 4140 & 416 for stainless. (Modern guns). Much weaker, free machining steels are generally use to make ML barrels. GM uses 1137 last I checked. And they’re generally not heat treated these days, (“tempered”) modern or ml , other than stress relief in barrels. They get their strength from the alloy. Barrels actually need to be soft .relatively. . A hardened barrel would be too brittle.
Have been a gunsmith well over 40 years and seen a lot. Extremely rare to see a barrel failure. Other than from obstruction. Seen a few from human error in handloading, wrong powder etc. Remington had some faulty shotgun barrels due to a cheap weak alloy and poor workmanship that were recalled. I’ve made a lot of parts, including complete guns. , proofed a lot of barrels. Most shotgun barrels have a safety factor of 5, but are proofed in Europe at 1.3-1.5x working pressure, depending on the country. The US has never had a proof law. And following a successful manufacturing design/material is a good safe method to follow.
Because of the questionable alloy in that barrel, I’d proof it. Not using “eyeball” . But with careful accurate measurements. But with black powder, I’d bet the farm it will be fine, even with heavy loads.

 

[russellshaffer]

I have made a few cannons by boring out solid stock. My proofing method for them was a double charge of powder under two balls. It was always uneventful as long as I remembered to put a sand bag over the barrel. About taking measurements, I assume that any expansion at all would be cause to scrap the barrel?

 

[brayhaven]

I have made a few cannons by boring out solid stock. My proofing method for them was a double charge of powder under two balls. It was always uneventful as long as I remembered to put a sand bag over the barrel. About taking measurements, I assume that any expansion at all would be cause to scrap the barrel?

Yes. Any dimensional change would be a fail for me. I never had one fail. I’ve been importing classic guns from Germany for 12 yrs. I went ahead and proofed some older ones, just for peace of mind.
An interesting piece on pressure and safety margin in 4140 chromoly type barrels. Not ML https://www.shotgunlife.com/shotguns/tom-roster/important-information-about-shotshell-pressures.html

 

[russellshaffer]

For brayhaven; What would you suggest as a 12 gauge proof load? I'll tie the barrel to a board and fire it before I do any other work on my project.

 

[brayhaven]

For brayhaven; What would you suggest as a 12 gauge proof load? I'll tie the barrel to a board and fire it before I do any other work on my project.

I would probably go with @150 gr of goex or 140 Swiss 2f and 2 oz of shot. Using over powder card, lubed fiber wad and a shot card. A jet-x type model rocket fuse in the touch hole should work well for ignition. Be sure to mark your measure points including the up and down points and get precise readings. Doesn’t hurt to shoot a couple proof loads. Put the fuse in before you drop the powder.

 

[Zonie]

A word of warning, if I may.

Proofing a barrel can damage the barrel in ways that aren't easily detected.
Internal cracks can be created and to detect these the barrel must pass a non-destructive test such as a Magnetic Particle Inspection. Not quite as good as a MPI is a Fluorescent penetrant inspection. It only tests for surface flaws but that is still much better than not doing any testing at all.

The mag-partical inspection can be done by companies which specialize in rebuilding engines. They use it to test the crankshaft for cracks and other flaws.

Before using a barrel that has been proofed I urge the shooter to have a NDT preformed. If that is not possible, IMO if the barrel is newly made from quality material (no welded "seamless tubing), it is better not to run a full load proof test.

If the barrel is very old, the Non-Destructive testing is almost mandatory if one wants to shoot the gun.

 

[voltigeur]

Do not forget that a barrel enlarges when firing and the breech plug can slide if the walls are too thin. The stresses in a barrel is not limited to the pressure as say previously. For me, the best thing is to test it after his construct or cut a long enough piece to fit a breech plug, put in it 3 times the normal load of powder and a patched ball.a 12 gauge is not a small caliber.