I don't know if it just applies to the base weapon damage or what, but I got a mighty missile launcher, and with 4 points in explosives and 2 in heavy weapons it only does 463, while a normal one does 460, which i'm no scientist, but that seems a tad bit less than 25% bonus damage.

The best I can figure is it only applies to the impact damage? Or it's just showing the wrong number similar to what rank 5 of big leagues does, which if it's the former then time to find a new legendary one!