While it is a distinction that no shotgun barrels were cold hammer forged, but for years, Ithaca "Roto-Forged" shotgun barrels for M37 pumps over a steel mandrel. The mandrel was inserted into a heated tubular billet and the billet was auto-hammered with 4 hammers roughly the size of flat irons with the force of 132 tons until the metal extended over the mandrel the desired length. The hammer marks were removed by a lathe.
(paraphrased from Walt Snyder's book, Ithaca Featherlight Repeaters).