Seamless tubing: safe?

[BillinOregon]

My guess is that there is a lot of controversy over the use of seamless tubing for making smoothbore muzzleloader barrels for folwers, matchlocks, handgonnes, etc. I know some well-respected makers are up front about using it for these purposes, while others swear they would never be caught using it for anything. What are the pros and cons of using this stuff? Are there various grades/specs/strengths?I am thinking it would be an economical choice for an arquebus or other pre-flintlock firearm, but I want to be safe. Obviously, a well-fitted breech plug is a given.

 

[bezoar]

Different grades and all, but the real thing is that most people want a big heavy duty THICK walled barrel if they shell out a 1000 for a gun.

 

[BillinOregon]

Bezoar: Understood on a $1,000 gun. I am hoping to build a very simple serpentine arquebus for under $100. That said, it occurs to me that, using 1-inch outside diameter tubing, I could sleeve the chamber area for added strength, and use a very short piece of sleeve at the muzzle for the belled effect. Safer yet, and still cheap.

 

[Zonie]

BillinOregon:
We had so much fun with the metallurgist question I decided to throw some wood on the fire with your question.

There is a formula called Barlow's Formula for calculating the Burst strength of pipes and tubes.
The formula is P=2St/D where P is the pressure, S is the Ultimate Tensile Stress of the material in PSI, t is the wall thickness and D is the outside diameter of the pipe/tube.

A rough calculation for 1018 Cold Drawn Carbon Steel Tubing.
1 inch in diameter, .219 wall (.562 bore) goes like this:

2X(58000 X .219)/1 = 25404 PSI.
That sounds pretty promising EXCEPT, there is a safety factor that must be delt with.
SAFETY FACTOR:
Steady gradually increasing pressure =4
Sudden change (0 to max) in pressure =6
Vehement pulsations= 8

I'm not sure whether the explosion of a charge of Black Powder is Sudden or Vehement, but because it is not thought of as a plusation, I will use the value of 6 in my calculations.

When you divide the 25404 for pressure by the number 6, you get the safe pressure, so for Sudden changes the pressure the answer decreases to 4234 PSI. (For Vehement pulsations the pressure decreases to 3175.5 PSI.)
IMO, these clearly are not good enough for a gun barrel.

If we decide to use good material like Aircraft Quality 4130, the Ultimate Tensile strength is 110,000 PSI.
Plugging this into the equation we get 2 X(110000X.219)/1=48180 divided by the safety factor of 6= 8030 PSI.
Even this in a .56 cal barrel is IMO, just marginal.

If we use heat treated Aircraft Quality 4140 with a Ultimate Tensile Strength of 145,000 PSI, we get 2 X(145000 X.219)/1=63510 PSI divided by the safety factor of 6 =10585 PSI.
This is the first pressure I would be willing to stand around when a load of Black Powder is touched off in the tube.

Some may say the "safety factor" has too much butt covering in it. Some may say they used some seamless tubing and they shot a 150 grain powder load under a double ball proof test and everything is fine.
I say, this is a matter of Life and Death. When the Life is mine, I would rather the next shot doesn't equal my death.

Notice: because different materials have different strengths depending on the alloy, the method of forming and the heat treatement, the Ulitmate strength varies a great deal.

In the Aircraft Industry, this is why millions of dollars are spent analyzing the materials and the operating pressures for designs of critical hardware. It is also why every stressed part made must have certification documents for the material the parts/tubes are made from.

For the home builder, obtaining this kind of documentation is next to impossible, and the scary part is, a piece of 1018 cold drawn tubing with a 58,000 pound ultimate strength looks just like the piece of heat treated 4140 so when the guy at the junk yard says "Sure this is Heat Treated 4140", how do you really know?

My recommendations is to buy a barrel from a known company who can do the analysis, and can assure the material meets their specification requirements.
If they do their job correctly, the Seamless Tube barrel may be just fine for its intended use.
For the thick walled hunk of tubing someone got from Bill's Scrap Yard, use it for a nice chin up bar for the kids. ::

PS: The numbers for the Ultimate Tensile strength came from a steel companys liturature. I note that their values are lower for a given material than I've seen in other sources for cold rolled bar stock.
I'm not sure if this is because of the process of drawing the material into the tube rather than cold rolling it into bar stock and boring it out is the reason or not.

Yes, I also have some barrels made from bar stock that are much thinner than .219 and they work fine but the question here delt with tubing, and its suitability for gun barrels, so that is what I confined my answer to.

 

[BLAHMAN]

That's interesting, how do they make seamless tubing?

 

[USMCsniper]

I to am planning on building an Arquebus. I have desided to use a shotgun barrel (from a single shot break open, like a H&R or Steven's), plug the breech, and drill a vent-hole
:imo:

 

[BillinOregon]

Zonie: Lots to think about in your post. I have heard that Early Rustic Firearms and Jackie Brown both use or have used seamless tubing for some barrel applications. Wouldn't this be a huge legal liability?

 

[BS]

I am sure that some of the old barrels were made out of ....what ever was available. Isn't that the whole idea of proof testing?

 

[Zonie]

BillinOregon: Please note, I did not say seamless tubing could not be used.
The formula I used, does oversimplify some things, and I believe that's why the safety factor is as large as it is.
Formulas which are much more complex exist, and I suspect the final answers these formulas give might not force one to use heat treated 4140 for a gun barrel. Especially a muzzleloading barrel designed for black powder. I would hope the suppliers who use lower strength materials have done their homework with these advanced formulas. I suspect they have because of the liability risks they face.

Then again, because the maker of the barrel has no control over what kind of loads are used in it, they often supply recommendations for maximum loads just to cover their tails. These loads are often very mild.
These Max Load recommendations will usually help defend them in the case of a burst barrel. :shocking:

 

[tg]

Hey Bill, I've got some old PVC pipe in the shop...want a hunk of it?...might make a good peestol barrel, if you want to be a peestoleero.

 

[BillinOregon]

Thanks Zonie.
tg: That's a mighty neighborly offer on the PVC, but I wouldn't think of using it unless it is Schedule 40. ::

 

[caneman]

Remember (If you are old enough) zipguns from automobile antennas for 22 shorts. We were just ahead of our time.

 

I built a gun the other day (took about a half hour) out of a 2x4 and a hunk of 32" half inch copper water pipe with an end cap soldered on. The lock was a simple hole in a peice of wood for cannon fuse. I fired it increasing the loads 10 grains at a time, I used a double patched .490 ball. I measured at spots with a micrometer and found no expantion. I was smart enough not to stand next to the thing when it went off. Now this is the weird part...1/2 water pipe is rated at about 400 psi, my gun took 110 grains of black powder before blowing up. My Lyman book puts that load at about 1400 psi.
Any idea what to make of this other that I am sometimes posessed by demons and copper water pipe is not the hot setup for a barrel?

 

[Stumpkiller]

From an outside observer: it seems that seamless tubing would be a better choice than seamed tubing. :hmm:

 

[BillinOregon]

Mercy sakes, Dread! I'll bet you gathered more than the usual peanut gallery at the gun club with that piece of ordinance. When I was a squirt, we used galvanized iron pipe with a screw-on end cap drilled for a firecracker fuse. Projectile was a marble and charge was one Black Cat. No blow ups.
Wonder how much powder that PVC the our brother Oregonian from McMinnville has offered would take before she lets go ...

 

I'm curious as to how the tensile strength of original damascus barrels compare to,.... the heat treated Aircraft Quality 4140 with a Ultimate Tensile Strength of 145,000 PSI??????

YMHS
rollingb

 

I'm curious as to how the tensile strength of original damascus barrels compare to,.... the heat treated Aircraft Quality 4140 with a Ultimate Tensile Strength of 145,000 PSI??????

YMHS
rollingb

:agree:

I also have wondered at the reasoning of persons that would refurbish some of the twist steel double barreled shotguns and shoot them without hesitation and them balk at the use of high tinsile tube stock.

Almost every barrel in use begins life as a form of tube stock! There may be a few exceptions, but I am sure I cannot afford them.

:results:

 

[Henry]

Have a look at the mortars used for fireworks ,
they are made of rolled cardboard , some calibers
forbidden in Canada and USA but in use in Italy
are up to 32 " diam. and shooting a 40 lbs of
explosives .... in a card board mortar .

As long as the inertia of the projectile is low
enough to get the thing moving before the pressure
is high enough to burts the barrel , all is well.
a copper barrel is OK , brass barrels were used
( much thicker I agree )

OK for smooth bore shot guns , but rifles
do develop higher pressures .

Now .... imagine that for some reason
the projectile stops it progression in the barrel.
The pressurized gas must go somewhere .

There are pictures of high quality shotguns
with a funny bulge of the side , and other pictures
of lesser quality shotguns with blown off barrels .

Also many XVIII th reports of barrels that
failed the proofing test .

Liberty is making choices , in much of Europe
you do not have this liberty , barrels have to
be proofed by law ( about 75 $ a shot ... pun intended )

 

[Joel Lehman]

On the Tennessee Valley Manufacturing web site, Jack Garner says his shotgun barrels are made of tubing. Over the years I'm sure he has sold plenty of fowlers using the tubing barrels. I don't think he would be selling them if they were not safe.

I ordered a 20 guage fowler from Brian Turner last week. He said he built most of the fowlers for Jack Garner for 12 years before going into business on his own. He uses the TVM hydraulic tubing barrel.

Joel Lehman

 

[AeroE]

I would like to make a couple of comments -

First about the formula above for calculating stresses due to pressure. The equation above is a form of the thin wall pressure vessel formula for the hoop stresses (circumferential stresses) in an infinitely long cylindrical pressure vessel. It is not applicable to rifle barrels, but it gives a good approximation in thin wall smoothbore barrels. The longitudinal stress due to pressure in one of these vessels is one half the hoop stress.

There is a more complicated method for thick wall pressure vessels that I don't have at the tip of my fingers, but a search of the internet will uncover it easily.

Infinitely long (or perhaps semi-infinite in some text books) means that it gives the wrong answers near the boundaries, such as at the very edge of the muzzle, or more importantly, at the breech, particularly where a breech plug is installed.

In application, here is the way I would use this formula:
1) Estimate the actual operating pressure, P.
2) Establish a Factor of Safety (F.S.) to use against the operating pressure. The purposes of a Factor of Safety are to accomodate variations in workmanship, variability of material properties, errors in the engineering methods, and uncertainty in the design load (operating load). In fact, for pressurized system design, I will use two factors of safety; one to cover for burst pressures or Ultimate Load where the part will be on the verge of catastrophic failure, and a smaller one that I will as proof pressures or a Limit Load case where I will insure the part does not yield or deform sufficiently to inhibit it's function.
3) Establish the allowable stresses (F) for the selected material, including a knockdown for operation at elevated temperatures (although this is a minor consideration in a muzzleloader).
4) For this example, I will use the thin wall pressure vessel formula in this form -
f = [F.S.*P]D/2t
f: hoop stress due to internal pressure
D: Outer diameter of the tube
t: Tube wall thickness
[F.S.*P]: the operating pressure multiplied
times the factor of safety.

5) If f is less than F, I will write a Margin of Safety, M.S., which must always be larger than 0.0 -
M.S. = F/f - 1 <= 0.0

Example: P = 20000 psi
F.S. = 4.0
D = 0.875
t = 0.080
Ftu = 100 000 psi (the ulitmate tensile strength)
Temperature: 150F,
strength knockdown on Ftu: 0.92
[I could look up actual numbers, but I'm not going to
for this tutorial since I didn't specify an
alloy.]
Hence: F = 100000(0.92) = 92 000 psi
f = [4*20000]0.875/2(0.080)= 437500 psi

M.S. = 92000/437500 - 1 = -0.79, i.e., unacceptable.

The wall thickness above is unlikely to show up in a rationally, or at least ethically eyeball, designed gun. Let's bend the applicability of the formula an examine a 0.375 wall thickness such as would occur at the breech where this high pressure occurs -
f= [4*20000]0.875/2(0.375)= 93300 psi
M.S. = 92000/93300 - 1 = -0.01, still unacceptable, but we know we are closing in on the correct geometry and should recalculate the stress with the correct formula.

Second, a comment about the material properties in manufacturers literature; they usually list "industry average" allowable stresses. This means they are pretty much as good as it's going to get and when the material test data is subjected to a statistical analysis, the allowable stresses that should be used for design are much smaller (in practical terms, it doesn't matter for products where lives or large finacial investments aren't at stake). Use caution. It is also extremely important to understand precisely what material you are using; substituting allowables from one alloy that you think looks similar to another is dangerous practice if you don't know what you are doing. Heck, sometimes it's dangerous when you do know.

By the way, I woudl be extremely cautious about using heat treated steels; in general, heat treating will sacrifice ductility, and in the case of gun barrels, ductility is your friend, helping to insure graceful material failures with lots of yielding, not sudden, catastrophic failures where parts break off and fly around.

On the topic of safety factors, FS=6 to 8 is extremely conservative in my opinion, and I don't know what the boiler codes specify, but I wouldn't be surprised if they are this high (there are several reasons large safety factors are required to insure safety in this sort of equipment; corrosion and others). For high performance aircraft, I use FS=4.0 for burst pressure analysis.

After all the calculating is finished, a proof pressure test of gun barrels is warranted. In the case of seamless tubing, I would use aircraft grade materials and magnetic particle inspect the tube for my own use. I would have to consider xray inspection if I was selling barrels.

And I would proof test the finished barrel with double charges and double projectile weight at least twice before I shot the gun; in the end, this is the proof of the part, not the engineering calculations which are simply a means of risk reduction.