Q2: Yeah, I thought so... I've been playing more narrative and hand-wavium games recently. In which backpacks allows you to carry more stuff, not just carry stuff.
About the weapon mass and ammo question, what I was trying to say was: do you add ammo mass to weapon mass, or do you just count the weapon mass to include that already. It doesn't necessarily matter, but my players now want to be as detailed as possible, and I wonder what the intention is - why they didn't want to play Spacemaster I don't know...
You may be right about the submachine gun, but it seems odd that an MP5 weighs more than a M16. The assault rifle holds 100 rounds, same as the submachine gun. Except the submachine gun has "lighter pistol sized rounds" (pp 107:HARP SF). For the record, the max weight I found for an MP5 on wikipedia was about 3.4 kg, but it was not noted whether or not it was loaded or unloaded. An assault rifle was, loaded, just under 4 kg.