1. Complete the game on any difficulty.
2. Fully upgrade the gun you want unlimited ammo for.
3. Go to the Bonus Features menu to “purchase” unlimited ammo for a particular gun.
4. Go to the Special Settings menu and activate unlimited ammo.
Note: when joining a co-op game over Xbox Live or the PlayStation Network, the host must have unlimited ammo turned on for your guns to have unlimited ammo.
How to unlock unlimited ammo for the Grenade Launcher
You can’t. This is presumably due to the fact there are several types of ammunition for it, and it can be purchased, unlike all other ammunition.
How to unlock unlimited ammo for the Rocket Launcher
You need to beat the game in under 5 hours on any difficulty. You don’t need to do this in one sitting - the game just adds up your best time for each level. You’d be surprised how easy it is to smash through levels once you have unlimited ammo for one or two guns.
How much to unlock unlimited ammo for each gun?
You don’t use money to purchase unlimited ammo, you use the points you get for completing a level. You can get up to 1500 points for an S rank, 1000 points for an A rank, and 500 points for a B rank on Amateur/Normal difficulty. I’m not sure if you get more on harder difficulties.
Needless to say, it’s going to take you a long time to unlock unlimited ammo for every gun in Resident Evil 5.