There are two hidden methods to obtain permanent unlimited ammo. In order for both of these methods to work, Rambo's combat knife needs to be at Level 1 (20 EXP points), which is difficult due to Level 2 being only 320 EXP points away, making the chances of the player collecting the necessary ammo before Rambo levels up slim. As such, these methods were likely only intended for debug purposes and not meant to be discovered by players. If you manage to collect exactly 2 throwing knives, 3 arrows, 4 exploding arrows, 5 machine gun rounds and 6 hand grenades, all ammo and health items will be permanently refilled. The other method to get permanent refills is to have an ammo count of exactly 77 throwing knives, 77 arrows, 77 exploding arrows, and 77 machine gun rounds.