Who needs an AR when you know how to use the old double-barrel shotgun? Also, a Winchester or Marlin levergun would do quite well in this application. I rather like my Mini-14 in this roll too, but I'm sure plenty of guys may or may not agree. It's all up to the operator to make it work.
If I were looking to change anything, that shotgun would have 20" barrels for better manuevering indoors. But like Clint Smith is quoted as saying about longer barrels indoors, "if they grab my gun, they better hang on because I'm lighting them up and it's going to be an E ticket ride."