I don't know exactly how it went down, but we know that opening doors with long guns can be tricky.
I sort of think that if someone was able to barge in, they would have been "toe to toe" with the home owner, would have knocked him back/knocked him over, or at least gotten a good opportunity to grab the long gun.
There certainly is a discussion about the decision to open the door - getting visibility to the outside etc... But I'm thinking that, if you are going to open a door, a long gun is not a good choice when doing that.
Either that or you just turn the door knob and immediately jump back with your long gun leveled at the people who just knocked...