Start a game in Normal Mode.
Every time you beat Nemesis you'll get:

1 time: Eagle Parts A
2 times: Eagle Parts B
3 times: 3 Heath Sprays
4 times: Shotgun Parts A
5 times: Shotgun Parts B
6 times: 3 Health Sprays
Every time: Assault Rifle

If this is your second visit in the game(you finished the game and loaded your save)you'll get if you've beaten Nemesis everytime instead of the Assault Rifle the Infinite Bullets.Use it on one of your weapons and you'll have Infinite Ammo then.