Heavy Bolters, like all weapons, have an ammo roll that needs to be checked whenever you roll a 6 to hit. They have an ammo roll of 6. Also, you roll up to 2 sustained fire dice when you hit. Each die has one jam "pip". So, if you roll a 6 to hit and choose to roll one sustained fire die and it comes up a jam, you would need to roll two ammo rolls of 6. If you fail either roll, you are out of ammo and the weapon cannot be fired for the rest of the game.
The upside is it is Strength 5 and does D4 Damage per hit!
Players can find ammo and other things during the game, so the chances are if you do fail an ammo check, you could find some during the game.
Hope this helps you.
John