You can unlock unlimited ammo for the following weapons by reaching the levels below. Note: These effects carry over into a new game and can be used once you collect the weapon.

Infinite Foam Ammo - Reach Level 30
Infinite Grenade Ammo - Reach Level 40
Infinite Rocket Ammo - Reach Level 50

Alternately, you can collect all packs for a weapon to unlock infinite ammo for it.