During game, go to the Inventory Screen and then press L2, then R2, then Triangle, then L1, then L1, then Circle, then R2, then L2 and you will hear Lara Sigh. Then exit Inventory but this only works in the PAL Version.
Enter into Lara's inventory by pressing SELECT then enter the following:
L1, Triange, L2, R2, R2, L2, Circle, L1
You receive 999 ammo for Uzi and Magnum, as well as 165 shotgun cartridges. You may re-enter cheat when ammo gets low to replenish.
Handy if you are having trouble killing T-Rex and bears in early levels with your pistols.