This, from another thread, is Edgar Spencer's very educated opinion on the HT over A stamp.
"Every alloy is defined by it's range of individual elements, but also by it's heat treatment. A set of Vulcan steel barrels may have the same chemical analysis as a set of Acme or Peerless barrels, but those high end barrels may have a much finer grain structure, achieved from very precise times and temperatures. Does HTA stand for 'Heat Treated, Annealed"? I don't think so, simply because annealing essentially yields a nearly dead soft, stress free material. It's just my guess that it might stand for Heat Treated Alloy Steel. Those heat treat cycles for C, Cr, Ni, Mo steels are Normalizing, to achieve hardness and tensile strengths, and Tempering, to achieve ductility. It does no good to heat treat it to a high tensile strength, and have it shatter like glass.
If one supplier had orders for tubes, ultimately to be used in V and A grade guns, he may have made them all from the same heat of steel, and maybe even the same heat treat lots. He may have simply been asked to certify a certain number or quantity of rough tubes. In our own company, some castings may have gone out the door at $2.50 a pound, and others, of the same metallurgical history, at $50.00 / pound. It's how much paper with signatures that went with it that made the difference."
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"I'm a Setter man.
Not because I think they're better than the other breeds,
but because I'm a romantic - stuck on tradition - and to me, a Setter just "belongs" in the grouse picture."
George King, "That's Ruff", 2010 - a timeless classic.
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