Where do you get the psg1 in Metal Gear Solid 2

In Metal Gear Solid 2: Sons of Liberty, one can obtain the Heckler & Koch PSG1 sniper rifle by returning to the Strut F Warehouse after meeting up with Richard Ames in the Shell 1 Core. He will have given you a Lvl 3 card, which allows you to open rooms you were unable to enter before. You will find the PSG1 in one of the rooms on the second floor of the warehouse.