The main attribute I'm wondering about is accuracy. I know all the weapon skills increase with use, though I don't know if it's per attempt or per hit.
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Could use some info on accuracy, thanks!
It is my understanding (from reading the source code) any shot that damages one or more enemy increases experience for both accuracy and the relevant weapon skill.
I
think that 'shot' in this context means single action, so if you throw a frag grenade that causes damage to two enemies, it still only counts as a single shot, and if you do a multi-round auto fire, it also still only counts as a single shot even if several of the rounds cause damage to aliens.
Hits using the sniper skill gains 1.5 times as much accuracy experience as the other weapon skills, but overall you get gain accuracy experience at a much slower rate than weapon skill experience (10-20% of the relevant weapon skill increase, depending on the weapon skill) The amount of weapon skill experience gained per hit depends on the weapon skill.
For each skill, there is also a cap for how much experience can be gained in a single mission.
I know using up TU's improve speed(but I don't know if it's per TU used or that I have to use up ALL TU per turn)
You gain speed experience for TUs spent on moving and firing.