A fixture for using letter and number stamps is described in one of the Metalworking Projects books. I'm pretty sure it was in one of the last four (volumes 2-6) which are still in print. It held the stamp in a fixture that went into the taper on a milling machine. The fixture was spring loaded and could be variably 'loaded' depending on how far down against the work the stamp was pressed. The desired stamp would be loaded into the fixture, located over the work, then pressed down by some amount to load the spring mechanism. It would then be raised to some height above the work and triggered, releasing the spring energy imparted during the loading step amd driving the stamp into the work. The depth of the stamp in the work was proportional to how far the stamp had been loaded during the 'loading' phase. I think with practice this fixture could produce very professional results. One would need to lay out the spacing and figure out the loading for different letters (an M would require more loading than a period to produce the same depth). I was looking at it to number micrometer dials, but it should work just as well on rifle barrels.