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What is the material these large bore,heavy powder load barrels are being made from?
Bob, If you are concerned about safety?--- It probably doesn't matter due to the fact they're shooting Black Powder. 12L14(the std. for BP barrels last I heard) has a yield strength of around 60K and 1144(Green Mountain is using this steel I believe) is up at 100K. One might bulge a barrel with a double load or compromise it if the walls are too thin at the rear sight, underlug etc. but doubt a properly manufactured and unobstructed barrel will blow up with heavy charges of Black Powder, It just doesn't burn fast enough to develope the kind of pressures needed. I shoot a 62 cal 1 1/16 straight(36in-1/72-.010 flat bottom rifled) Sharon barrel made of 12L14 with 120 grains of 3f as my std. charge for deer hunting. I have tried heavier loads but it blows the hammer back and I really don't gain that much in energy/velocity. I built this rifle in 1978(see pic).
Also FWIW---I have a 45 smokeless muzzleloader that shoots around 2700fps with a 200 grain saboted 40 bullet and it is only developing around 35000psi based on pressure traces---It is built with 4140 std. barrel steel however 
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The yield strength of 12L14 is a totally meaningless number when its used as a gun barrel.
12L14 when used as a gun barrel is brittle. Brittle is what makes the numbers meaningless. The cold rolling that makes the material brittle also increases its yield number. But they do not test for yield when subjected to internal pressure and brittle steels fail shock tests. Gun barrels are shock loaded.
ALL cold rolled steels are brittle this is why they cold roll them. Brittle steels, in general, machine easier (if anyone wants a demonstration turn some cold rolled on a lathe then anneal it then make some more cuts. It will be more difficult to cut. While its possible to anneal 12L14 and make it more ductile this does not get rid of the high levels of Lead and Sulfur and Phosphorus which cause inclusions and flaws in the steel. This make the steel even if annealed more failure prone than a similar alloy without these metals added.
THEN we have the fact that the stuff is a cheap steel with little care taken in its manufacture compared to a certified quality steel. So its has things like fissures and inclusions etc etc.
We have to ask why would someone make a gun barrel from a material that the a steel maker, decades ago, stated was unsuitable and specifically stated that it should not be used when
Wrought iron? GOOD wrought iron is less likely to fail than cold rolled steel. The Springfield Rifle Muskets ALL have skelp welded iron barrels. Some were converted to 58 Berdan breechloaders. They are still a reliable barrel when in good shape. Wrought iron is a very TOUGH material. It has sufficient strength for a ML barrel using round balls or light charges and a bullet like a Minie (the Springfields were proved with 280 gr of Musket Powder and a Minie spaced 2" off the powder IIRC).
This begs the question of why the Gov't used iron when steel was available. However, control of the alloy was pretty crude and it might be that fluctuations in alloy might make iron a better choice.
Low quality iron, and lots of barrels were made of low quality iron in the first 1/2 of the 19th century and beyond, was far weaker due to high levels of inclusions.
Cold rolled works well for many things, but its not good for tubes bearing internal pressure and shock loading.
Claims that the barrels never fail are simply false. Failures of 12L14 barrels, I am told, is why Douglas stopped making ML barrels. I know there were failures and I know they stopped making ML barrels. Douglas is not the only maker who have had barrel failures. Buts its something nobody wants to address.
Dan
