Doubles the ammo you spawn with. Does not increase the max capacity of the ammo you can pick up. For example, u spawn with 500 elec ammo, 250 base ammo and 250 from the pack. You cant pick up ammo from kills untill you're below 250. You cant pick up more then 250 ammo on the field. If u spawn again (enter and spawn) You're back at 500.
This somewhat decreases the value of the ammo pack in some situations. It's often handier to have other items such as disruptors or a relocator with you instead of the ammo pack.
Ammo pack is very useful for charging into enemies while blasting away with weapons such as pulse cannon (as nomad). In defensive situations, such as when humans only have their base left and beast is laying siege to it. In that situation, nomads often dont live long enough to pick up the ammo from kills. And you dont want to run out of ammo while being surrounded/raped by five predators. If done well you can take out two-three enemies before they take you down. Aim for the summoners first, they usually take out the shields, which is bad. Take out the healers before they can ressurect/heal/shield stuff. Then take out the weaker support units, such as scavengers with ranged weapons. Evade the melee swings from the predators that are chasing you. If they frequently use wind shield, then take repeater or flux. Flux does not require an ammo pack.
Ammo pack is also very handy for killing a behemoth on your own. No weapon has enough ammo to take a fullhealth behemoth out without ammo pack.