If this were to be in game I think the following stats would be in order.

• Can Repair but not Upgrade friendly buildings
  
• Reduced Range (Maybe a little over melee range)
  
• Can burn enemies (Only for the duration that they're in contact with the gun.)
  
• +15% Firing Speed (Fires simular to Pyro's flamethrower)
  
• -50% ammunition

This could be a secondary for a Engineer to use when he's out of Metal. The downside is he has no range except the sentry gun. (No wrangler or pistol) But this allows the Engineer to use another reserve dedicated for healing buildings.

This weapon would be slightly situational in loadouts. Personally for a Combat Engineer who doesn't really care for the faith of his sentry gun I'd stick with the handy pistol. In some cases I like to put my mini sentry in a "good" spot and camp it, then I'd use this gun. This is for those people who nest up in tight areas that don't have spots where range can easily eliminate you.

This is a reasonable secondary weapon and it seems roughly balanced considering other options for the Engineer.