Avi sez, "AFOL Shannon Sproule built this charming Raygun entirely from LEGO parts." Shannon calls it the Russian Tokarev TT-34 Atomiser and notes, "Every mechanonaut was issued with a Tokarev laser pistol. They were small, lightweight and proved very reliable on the lunar battlefield." As this implies, there's a whole contrafactual mythology that this belongs to, called Battle for the Moon.

Russian Tokarev TT-34 Atomiser (Thanks, Avi!)