After beating the game on Veteran, Professional difficulty will unlock. You will also have the ability to unlock unlimited ammo in any gun you have completely upgraded. You must purchase this ability with your "exchange points" which are given based on your grade in each chapter, and it only affects the specific guns you've used your exchange points on. The feature must also be turned on in the Bonus Features menu.
If you want to take that even further and unlock unlimited rocket launcher ammo, you can replay the game and beat it in under five hours (obviously easiest on Amateur difficulty).