

Part of it is the holes are fairly uniform, but the big tell is no “flaps” poking outward. A drill removes material as it drills, so only a little is left over to flap out when the drill pokes through. A bullet would have to push ALL of the material from the hole outward, which would be a lot of material for a one inch thick hole.
From the photo it looks to me like there is some material being pushed out the front from the back, so unless you know of a way that a high speed bullet moving toward a metal sheet would pull material backward as it moves forward then it would be hard for it be shot from the front.