While not a traditional cheat, this glitch‑like technique can give you a tactical advantage. To crawl where enemies cannot shoot you:

Direct input button cheat codes for infinite health or ammunition are not a native feature of the game. However, players can unlock massive advantages, hidden missions, and top-tier gear through targeted gameplay achievements. 🔓 Major Unlockables and Secret Missions

We can also dive into a complete guide on how to on your PSP, or explore the exact locations of all hidden intel documents across the campaign. Share public link

If you only care about using every weapon in online multiplayer, there is a simple trick that bypasses all the unlock requirements. According to Neoseeker’s cheat print page:

These features can be unlocked through standard gameplay and specific milestones: Tea and Cupcakes Mission : Complete 50 Custom Missions

If you want, I can:

These are not official menu cheats but gameplay exploits using button sequences that function like cheat codes.

: Access the loadout screen before deployment to swap attachments. Use suppressors and laser sights for nighttime stealth operations, or heavy barrels and high-capacity magazines for loud, direct assaults.

: Unlocking the "Tea and Cupcakes" mission also allows you to select Maids as enemies in Quick Mission mode.

Awarded for racking up a massive number of kills. (Perk: Increases your overall weapon accuracy).