try attaching the ammo_pickup() to a key

in your main you would put on_w = ammo_pickup;

this assigns the w key the function of "ammo_pickup"

if you need to call a function you wrote off this, you could type

function give_ammo()//or whatever name you want
{
my.ammo_pickup();
}

then assign the w key to it in your main

on_w = give_ammo