gotcoalman posted how he fired through an ~8mm steel H girder at the local quarry twice with his .44 Kentucky rifle and a patched round ball. He said that the only other gun there that could do it was a .30-30.
When he posted about it folks didn't want to believe his claim so he set out to take a video of his gun firing through an 8mm [.31 inch] steel plate. He loaded it with 100 grains of 777 and a patched .433 round ball and guess what?
Well, check out the video and see for yourself. BTW, he dislocated his shoulder in the process and his son took a piece of shrapnel in the leg to accomplish this video from 3 different angles.

http://www.youtube.com/watch?v=0Phjzjk0f7Y