ProjectileImmunityFix

In Minecraft, when an entity is damaged, they get a short invincibility period. However, lots of weapon mods (or gun mods) don't consider this when making their weapons, meaning rapid-fire guns are doing significantly reduced damage to the target.

This mod makes it so projectiles of all types no longer give invincibility frames. 
An example of this functioning is with Multishot - with the mod installed, all three arrows can hit the target, dealing three times the usual damage (which, at least in my opinion, seems like it was intended by Mojang, since without it, Multishot kind of sucks.)