Rabbit,
While mild steel is acceptably hard enough for a dent raising plug, it machines like crap unless it is a leaded alloy like 12L14 or a tradename alloy like "Leadloy". Basic mild steel like 1020 or such has large grain structure and kinda tears when machined. So, I simply use a fine grained tool steel like O-1 "drill rod" and it machines to a fairly fine finish easily in he normalized condition as purchased. Brass could be used also as I believe Dr. T points out in his little book.
I simply turn the plug about 2 1/2 - 3" long, to the diameter of choice, drill a 1/4" hole about a 1/2" deep in it while in the lathe, then cut it off and weld it to a 1/4" rod of appropriate length, then generously round (or angle) the nose on a belt sander and polish the nose and o.d. on a hard Scotchbrite wheel. Use a thick grease on the plug before you drive it under a dent. Use a small steel hammer of 2 -4 oz weight with a polished face to tap or a polished brass hammer if not planning on refinishing the barrels. I've thought of trying some Mylar film between the hammer and barrel when refinishing is not the plan.
