Vague questions there. It sounds like you already answered them tho. The best thing to do is go rent as many firearms as you can test them, and look over them yourself instead of asking opinions, because you will get a different one from every user on this site.
You should narrow down what your looking to get by giving a price range for both the handgun and assault rifle. Otherwise, you'll get suggestions like, a Colt 1911 E1 Presidente for a nice 45, and a Les Baer Super Varmint for a nice assault rifle. As for the 870 tube extension, I dont know, look some up and find out the specs on them, and do some math.