Another thing is to keep the firearm out of sight to the person who is at the door. Keep it behind your back but in your hand.

That said, the whole "authoritative" knock on someone's door is generally rude and offensive to begin with and criminal home invasion types often copy it. That is one of the reasons I figure that Dog the Bounty hunter is eventually going to get shot by someone when he and his black leather clad rudeness circus go barging through an apartment building.