I don't know how the heal works.. Never played on a rescuegun file but I think you can't just compare the Rescuegun with a Melee, because Rescuegun can actually hit a Team Mate in order to heal him, a Melee doesn't hit Team Mates so this is the first thing you need to do... And if the Heal is treat like negative Damage (I don't know if it is like that.) then it's easy enough to make the Damage positive

I'm interested now I'll take a look