Not sure if any of the devs ever shot a firearm at all, but in case they haven't: NO, sandstorm, rain, fog and other weather conditions don't magically change a bullet into a BB pellet... This is the most annoying feature of the game, as I tend to have about 80% of battles in bad weather (should I R&R instead then? but that would require prior knowledge of the weather, like a forecast or something).
Make it impact visibility and to-hit change, but come on... a bullet in the rain is as lethal as a bullet on a nice sunny afternoon...
Now, to the point: it wouldn't be a problem in general, but the AI simply can't handle it and NPC are absolutely stupid and don't take weather into account: If you know you'll only graze an enemy, you won't run head-on vs 5 enemies relying on your HP and assuming you'll kill them one by one (which is what my *brilliant* allies tend to do).
It's really annoying that I am guilt-driven to abandon good tactics and sacrifice my mercs' health to save those stupid NPCs...