The choice between shotguns is pretty much down to personal preference since the 870 pumps so fast anyways. Again between buckshot and flechette it depends on what sort of areas you tend to patrol with your shotgun. Personally I don't really use the shotgun for too many maps apart from Grand Bazaar and Operation Metro. I'll sometimes use it if I want to full out bitch mode defend a bomb site but that isn't usually my style. I don't know if adding a laser sight would tighten your spread on a shotgun though since the spread comes from the nature of the round itself. I may be wrong though. I agree that frag is pretty terrible and I guess the only use would be if you were using it with SUPR but whats the point of surpressing people up close when you can just kill them....
As for using the engineer weapons as assault rifles, well they are carbines which is essentially a shortened assault rifle anyway.
As for using the engineer weapons as assault rifles, well they are carbines which is essentially a shortened assault rifle anyway.