[@AdvancedJ3lly]

none of the heavy weapons let you start with a backpack supply. they seem to stat it as loaded from an ammo box. 

ex. according to 'Only War' core rulebook the heavy bolter has 60 rounds per clip. Probably from an ammo box and belt fed to the bolter. the heavy stubber has 75, again probably from a box carried by the comrade. 

Though with the heavy stubber, that one has around.... 4-5 variants if you pull from books like rogue trader, and dark heresy. Ranging from a 40 round drum mag stubber to a 200 round backpack and several other sizes inbetween.