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).
Here's Mr. Snyder's Shotgunworld post on the topic:
"A mandrel was inserted in a 10 inch round stock piece. That went into the forging unit where it was heated red hot by induction coils. Several hammers than rotated around the blank stretching it into a 30 plus inch barrel. The hammer marks were ground off via a lathe operation, and then on to the rest of the barrel making operations. I tried to cover all this with photos in my book on the M-37." 
