[quote=@Ollumhammersong]
[@AdvancedJ3lly]

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

[/quote]

Woops, I'm confusing myself. Hot-shot lasguns/pistols have required backpack supplies, others are optional. 

Edit: The Errata flat out says specialization weapons come with 3 loads of ammo.