Tamid, American gunmakers chambered 12 ga. guns to 2 5/8" as that was the standard 12 ga. cartridge length until industry standards changed to 2 3/4" in 1929.. L. C. Smith was the only maker, to my knowledge, that chambered their twelve bores to 2 3/4" earlier, circa 1910. Everyone else chambered at 2 5/8" until 1929. As an aside, the so-called 2 1/2" chambered English guns, almost always actually measure 2 5/8". Sandlapper