missiles BBox are too big
Here, we can see a blaster's missile bbox:

I think it's way too big for the size of the weapon or the visual blast effect. This also applies to pulse rifle, but not to lucifer blasts, flamer flames (although the flame is under the bbox, which is quite weird too) nor to alien blobs.
I think this is one of the reasons it's this easy to damage friendly things with those weapons.
An obvious solution for projectiles is to add bounding spheres. They'd obviously much better encompass projectiles (in fact, they'd provide the exact bounding volume, since the projectiles are, themselves, spherical), and are super easy to cull against or check for collision. I'd like to see the bounding volumes be more generalised so we could gradually add OBBs and hierarchical volumes as well.