Basically, projectile weapons are ones where the bullets have travel time and trajectory to reach their target. Hitscan is when the bullet has no travel time and, when in the hands of AI, is essentially a flip of the coin whether you will get tagged with a bullet that fluctuates with distance from the shooter/target. It's fake and lame and would be a lot more believable for all guns in the game to behave the same way regardless of who is firing them.