WhateverProjectile:onCollision. Here's an awesome snip of code describing what i'm saying, from one of my scripts:
function ShovelProjectile::onCollision(%this, %obj, %col, %fade, %pos, %normal) {
dowatever();
Parent::onCollision(%this, %obj, %col, %fade, %pos, %normal);
}
(Inside package!)
Oh by the way, this should go in Coding Help.