It's possible that the steel of the drums is more cheaply made now. Everything seems to be. Perhaps older drums were stronger than they needed to be and as a cost cutting measure the engineers thinned the rolled steel they're made from. The only experience I had shooting at 55gallon drums was at a range and ball ammo was the only thing I used. It's hard to imagine that with the same materials and velocity, the .45acp bullet is penetrating better on the same test medium.
