p. 159 DW states that 'backpack supplies hold 100 rnds for Plasma, Melta and Las weapons, 250 rnds for SP or Bolt weapons and 75 shots for Flame weapons."
Furthermore, it states that "backpack ammo/power supplies are automatically issued with any heavy weapon."
Now, I think the ammo capacity for the SP and Bolt weapons is fine, no problems there.
My problem is applying that to Plasma Cannons, Lascannons and Multi-meltas. I think that's just too much ammo.
My proposed house rule is that backpack ammo supplies provide the listed ammo capacity for SP and Bolt weapons only; for wall other heavy weapons, they simply double the heavy weapons listed clip size. Furthermore, backpack ammo supplies can be used only with heavy weapons. The Soundstrike missile launcher may not benefit from a backpack ammo supply, as it already incorporates an expanded ammo capacity thanks to similar functionality incorporated into its design.
For example:
Plasma cannon would have 32 shots, Lascannon would have 12, a Heavy flamer would have 20, etc.
The thing is, once you get into an actual mission, you simply don't need that much ammo anyway. Halfway through Extraction, my players had used only 3 missiles and 40 heavy bolter rounds. I also like the idea of forcing my players to think tactically as to how they'll use their heavy weapon resources; I don't want them to just blast away willy-nilly with a lascannon because the RAW say that it comes with 100 shots.