Other Stuff like Diet Coke and Mentos?

That`s almost right.
You would add the gun and the bullet. However, the product of the bullets and head would equal how many bullets you need. So, it`s Bullets Needed=B(h) and then that would be G+BN.
That would equal a gun with enough ammunition to take on any number of heads.

However, you would also have to add a human head for anything to happen, so it`s G+BN+BN