Never having been able to get a nice, even, aligned stamp on a barrel I decided to make a fixture to hold the stamp while striking with a hammer.  Since virtually all the stamping done is on the breech end of the barrel the fixture was made as a feature on the bushing used to hold the barrel in the barrel vise.  My vise uses 1.750" O.D. bushings.  I made these out of brass with a bored I.D. to match the barrel breech diameter, slitted one side and milled a slot opposite the slit.  Slot width is only wide enough to insert the type slugs protruding from the handle.  A flat was milled over the slot to thin the bushing wall enough to allow the type slugs to contact the barrel. 

In use the bushing is slid over the barrel with the slot positioned over the area to be stamped.  This assembly is then clamped in the bench vise equipped with padded jaws while being supported underneath by a block of wood.  The type slugs are installed in the handle and inserted into the slot.  The handle is then rocked to make sure that the type is sitting flat on the barrel then struck with a hammer.  It's almost foolproof.

FWIW