so redv, I understand you tried to implement that in the ruleSet, but it returned an error? I'm unfortunately not very good at ruleSet editing, so I can't halp you. perhaps somebody else can?
having this option makes complete sense, as I said, but still it needs to be an option - "vanilla" openxcom should be as close to the original as possible, at least from the game mechanics like what weapon uses what kind of ammo.
also, now that we can disable craft weapons temporarily by clicking them, elerium conservation shouldn't be a problem, because you can choose to switch off the plasma beam and use avalanches instead, and if they run out, you can turn on the beam to finish the UFO off.