Added a condition statement to check if the remaining ammo is less than 1 second, and if so set ammo to 0 instead of possibly a negative value which can cause underflow.
I wasn't able to replicate the underflow myself, but I might as well be thorough.

I also realized that the ammo will decrease by 1 even if I tell the script to lower it by 0.5, so I edited the script to say 1 since it'd be misleading to claim that it decreases by 0.5