Infintite Ammo for any weapon in the game can be unlocked after meeting these prerequisites-

- Have that weapon's stats all maxed out
- Have beaten the game on any difficulty

After you've met these requirements you can purchase infinite ammo for that weapon under the bonus features menu. The only thing I can think of is that you missed one of the Infinite ammo toggles. You actually have to turn it on in two places. Once in the special settings where the costumes and game filters are and then again right before you start the actual stage you're doing.

- Infinite ammo -

1.) Complete all chapters of the game from start to finish on any difficulty.
2.) Fully upgrade the gun you want infinite ammo for.
3.) Purchase the "unlock infinite ammo" option for said gun.
4.) Turn on infinite ammo option in the Special Settings menu
5.) Start any type of game and turn on the Infinite Ammo toggle.
6.) Pwn.

Note: This goes for every gun in the game except the rocket launcher and grenade launcher. To get infinite ammo for the rocket launcher beat the game under 5 hours. The grenade launcher has no infinite ammo option, probably because you can buy ammo for it no one knows.

You must first complete the game to unlock the bonus features unlimited ammo feature. Then proceed to upgrade (or previously have upgraded) a gun fully. Once upgraded it will allow you to redeem points in the bonus features menu. Turn on the feature in the bonus Features menu, then start your game. A new option saying "unlimited ammo" should appear under the "attack response" option. Make sure this option is set to yes to allow infinite ammo.

Go to special settings where it says costume, filter select, and infinite ammo go to infinite ammo and turn it on there.

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Did u upgrade the gun fully and did u play with ur partner or online because if u played online infinite ammo might of been turned off