Stubber is a name used in WH40K to describe "our" firearms, with rifled barrels, from Stubb Gun/Autopistol (pistols, revolvers, UZIs) to Heavy Stubber (.50 cal HMG-like). Autogun is also a Stubber and is an equivalent of an automatic rifle. Their performance seems to be close to their Laser equivalents (better ROF, lower damage), but Lasers win out by superior endurance and ease of maintenance; they're, however, more difficult to manufacture.
Shotguns, however, always have been a separate type of weapon. SM Scouts use Astartes Shotguns, which are big versions of standard Shotguns used by eg. Imperial Guard or Adeptus Arbites.