Rifles make an extraordinarily clean hole, even in metal. I was amazed the first time I shot a .303 at an intake manifold.
No the intake manifold was not attached to an engine at the time.
Shotguns definitely give you a ragged edge (probably requiring a rasp for initial cleanup) but have the advantage of instantly and infinitely adjustable hole size.
Years ago, I was doing a job out in the Ca desert (enviromental clean up) on an old 50's/60's JPL site for testing jet engines and I guess rounds. The 6"-8" holes thru the 1 1/2" plate steel, were very clean! The concrete bunkers were sortof a hint.
