Some assumptions i make that may or may not be true:
- Weapon mass includes a full magazine
- weapon mass includes any backpack, belt or wrist mounted power source
Spare ammo: If you are talking about spare, as in one reload, it is fairly negligible. For example, an autopistol has a mass of 0.5 so the ammo, to me, would be a bit less and negligible.
power pack weight: Even if you agree with my assumption, there should still be a way to subtract the weight of the power pack since some of the weapons, like the Laser Rifle, specifically describe limited usage without the backpack. Also there may be times when a power pack is carried such as to resupply the front lines, and weight is needed for determining encumbrance. I agree that at least the backpacks should have mass even if the belt and wrist power supplies are negligible.