FLC AFTER LNDG AT CTAF ARPT WITH TWR CLOSED; TAXIES INTO MIL RESTR AREA.

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WE WERE IN THE PROCESS OF DELIVERING AN ORGAN. CTR CLRED US FOR A VISUAL APCH AT ZZZ TO RWY 18. CTR THEN CANCELED OUR IFR FLT PLAN AND THEN WE WERE TOLD TO CONTACT THE LCL CTAF FREQ. ALSO; BEFORE CTR GAVE US PERMISSION FOR A VISUAL; WE LISTENED TO ATIS. ATIS HAD INFORMED US THAT THE S SIDE OF THE ARPT WAS CURRENTLY BEING USED FOR MIL OPS. ON THE CONTRARY; MY CAPT MAY HAVE BEEN A LITTLE CONFUSED ABOUT THE INFO WHICH WAS BEING BROADCASTED BY ATIS. WE LANDED ON RWY 18 AND AFTER OUR ROLLOUT; WE BACK TAXIED FOR AN APPROPRIATE TURNOFF TXWY. WE CLRED RWY 18 BY MAKING A R TURN ONTO THE TXWY. AT THAT TIME; I WAS VERY FAMILIAR WITH MY SURROUNDINGS AND I CONTINUED TO HELP/ASSIST MY CAPT AS ALWAYS. FURTHERMORE; DUE TO MY PAST AVIATION TRAINING AND MIL BACKGROUND; I KNEW THAT WE SHOULD NOT TAXI ACROSS THE PARKING RAMP OF THE MIL. THE CAPT; HOWEVER; PROCEEDED TO TAXI TOWARDS THEIR RAMP. I THEN INFORMED THE CAPT THAT WITHOUT PERMISSION; WE COULD GET VIOLATED AND THAT THE PROPERTY BELONGS TO THE UNITED STATED MIL. HOWEVER; THE CAPT WAS UNDER THE UNDERSTANDING THAT BECAUSE HE SAW A SIGN THAT SAID RAMP; IT WAS OK TO USE THE MIL RAMP TO TAXI TO PARKING AT THE CIVILIAN FBO. I THEN FIRMLY; BUT TACTFULLY; STATED 'WE COULD GET AN FAA VIOLATION BECAUSE WE WERE NOT CLRED TO GO ACROSS THEIR RAMP.' HE CONTINUED TO TAXI STRAIGHT AHEAD AND PAST THE NEXT TXWY; ONTO THE MIL RAMP. AT THAT MOMENT AND BEFORE I COULD EVEN BLINK AN EYE; SECURITY POLICE (SP) APPEARED OFF OF OUR R WING. I INFORMED THE CAPT OF THE SECURITY POLICE'S PRESENCE AND WHILE TAXIING; WE CAME UPON A CONCRETE TYPE WALL OR BARRICADE; THAT SEPARATED THE MIL RAMP FROM THE FBO'S RAMP. THE CAPT THEN TURNED THE ACFT AROUND AND STOPPED IT. THE SECURITY POLICE CALLED THE ARPT MGR/AUTH. WHILE I SAT IN THE ACFT; THE CAPT TALKED WITH THE SECURITY POLICE. THE ARPT MGR/AUTH THEN ESCORTED THE CAPT AND I OVER TO THE FBO. CAUSE OF TAXI INTRUSION: THE ARPT DIAGRAM DOES NOT DEPICT THE FBO AREA AND MIL PROPERTY BEING SEPARATED BY A BARRICADE. ALSO; THE ARPT DIAGRAM DOES NOT DEPICT THE LOCATION OF THE FBO. THE TWR WAS CLOSED IN OUR CASE. CORRECTIVE ACTION: I WOULD LIKE TO SEE MORE DETAIL ON THE ARPT DIAGRAM; IN RELATION TO AREAS THAT CIVILIAN ACFT SHOULD NOT OPERATE IN OR AROUND; WHERE MIL AIR BASES ARE CONCERNED.