Cold rolled free machining steels are not approved for or suitable for pressure vessel use. And yes gun barrels are “pressure vessels”. This from a letter from LaSalle steel. Douglas used 12L14 cold drawn octagonal bars for ML barrels, even though they should have known better. They stopped doing ML barrels as the failures stacked up. In reality a bar of good quality iron makes a safer barrel that 12L14. Its ductile not brittle and will bulge and not break this also applies to steels like hot rolled and heat treated 4150 (41v50 is mil-spec for small arms barrels the vanadium) give longer life. 12L14 by the way is called “leaded screw stock”. For use on automatic screw machines. The next time you run a wood screw in a stock etc and it simply twists off at a very minor torque level remember the stuff is full of flaws. Free machining steel are free machining because at the mill they are (in the cae of 12L14) made with additions of lead, sulfur and phosphorous. These metals then cause inclusions in the steel that can cause failures for various reasons.Then it is embrittled by cold forming because “brittle” means the chips break easier. Gun barrels blackpowder or otherwise should be made of HOT ROLLED steel of at least CERTIFIED gun barrel quality. The certification means the steel is inspected at the mill and is certified to have no more than a certain level of flaws. Mill run steels are just that. Mill run. It is impossible to make any of the certified grades, gun barrel, aircraft, nuclear in s furnace used for mill run steels. It would then be contaminated. Nor can they be ordered, SFAIK from the mill in less than furnace melt quantities so small makers have to pool orders to get the tonnage up. So no 12L14 and a host of other cold rolled steels are no safe as gun barrels. Also the tensile number for cold rolled steels are meaningless when subjected to INTERNAL PRESSURE. This info all comes from, Jim Kelly, RIP, a metallugist who was into MLs and specialized in failure analysis . He wrote a number of articles for the old “Buckskin Report” magazine in the 1970s and 80s on this and I had mail contact with him as well. The only thing that has saved ML makers and the barrel makers from being sued out of business is the “handloader defense” where it is impossible to prove that the gun was loaded properly. In one case a faulty (as proven later by a friend) chemical test that “proved” that smokeless was used. Even though BP fouling right from a new can of GOEX gave the same “proof” in my friends repeat of the chemical analysis. All modern steels are designed for specific purposes. !2L14 is for low end hardware store quality screws. Forgive the typos I am supposed to be doing something that will not get me roasted on the WWW.
