I've advocated for either increased shotgun damage, an SMG, or a machine pistol since the first Xenonauts. That said, the shotgun should be great inside buildings and devastating at really close range. 00 shot from a shotgun has the same hitting power as a 9mm round and a single shotgun shell holds nine of them. So, going strictly by "reality" it should be like unloading an entire pistol at the target instantly. The other two weapons are basically just fully automatic pistols with the SMG having the advantage of being easier to control and having a larger clip. The machine pistol does have the advantage of being a one handed weapon, but would nearly useless at anything beyond very short range.