I tape a piece of steel measuring tape on the barrel. Grip the barrel very firmly in soft jaws in a vice. I have the designation written out on a piece of paper. I am right handed and right eyed so I stamp the last letter first and work backwards sighting down the right side of the punch at the previously stamped letter, resting the edge of the letter on the edge of the steel tape. With a little practice you can do a very nice job. You can polish the raised stamping using 220 grit wet or dry as you would buff shoes with a cloth and restamp to get them even or leave then slightly raised.
Here is a picture of the completed stamping on a stainless barrel. I have an MM stamp made up smaller then the numbers and I have a DRS stamp made in one piece. I mark all my barreling jobs with DRS.