Here are my picks for multiple categories:
revolver: Chiappa Rhino
Semi-auto handgun: Mauser C96
Lever action rifle: Henry repeating rifle
Pump action shotgun: either a Winchester 1897 or Kel Tec KSG
Semi-auto rifle/carbine: either a P90 or SKS-bayonet-extended.jpg)
Submachine gun: Sten gun
Of course, you can also go a totally different way with it, and model rather simple or typically plain guns that have gone through custom beautification, like ornately inlaid flintlock pistols, expensive handmade double-barrel shotguns or intricately engraved 1911s