WRONG ARPT APCH FINALLY RECOGNIZED AND PIC MAKES A MISSED APCH.

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IT WAS A CLR DAY GREAT VISIBILITY. WE MISTAKENLY MADE AN APCH TO MALMSTROM AFB INSTEAD OF GTF. WE MISTOOK MALMSTROM AFB FOR GREAT FALLS ARPT. REASONS THIS OCCURRED: 5 MI DISTANCE BTWN ARPTS. BOTH MAIN RWYS 10000 FT OR LONGER; APRONS AT BASE AND SECONDARY RWY AT GTF VERY SIMILAR BUT MIRROR IMAGE. NO CAUTIONS OR 'NOTES' ON ARPT DIAGRAM TO ALERT PLTS OF POSSIBLE CONFUSION. ARPT DIAGRAM OR APCH PLATES SHOWED WHERE CITY WAS LOCATED MISTAKE WOULD NOT HAVE HAPPENED. WHEN WE THOUGHT IT MIGHT NOT BE THE CORRECT ARPT CONSIDERABLE DISCUSSION TOOK PLACE; KEEPING OUR HEADS IN THE COCKPIT MORE THAN NORMAL REVIEWING ARPT DIAGRAM AND APCH PLATE FOR VOR RWY 21 APCH. THE DME AT GTF WAS NOT A CONCLUSIVE REF DUE ITS LOCATION (1.6 DME) SW OF ARPT; ADD TO THAT A 2 MI RWY; 7 MI FINAL APCH AND THE 10 MI RADIUS COULD BE BELIEVABLE AT MALMSTROM AFB. NO PRECISION APCH ON RWY 21 GTF. IF AN INST APCH HAD BEEN FLOWN; THE PROBLEM WOULD NOT HAVE OCCURRED. GUARD RAMP AT GTF MADE MIL ACFT AT BASE NOT OUT OF PLACE. ATC DID NOT SAY ANYTHING UNTIL GAR HAD ALREADY BEEN INITIATED. IT WAS DISCOVERED BY; RAMP AND BUILDINGS ON WRONG SIDE OF RWY. XCHK OF VOR APCH AT GTF. DIFFERENT VASI SYS ALTHOUGH IT WAS ON L SIDE OF RWY JUST LIKE GTF. WHEN THE MISTAKE WAS DISCOVERED WE EXECUTED IMMEDIATE GAR AND PROCEEDED TO GTF FOR UNEVENTFUL APCH AND LNDG. HUMAN FACTORS: PERCEPTIONS JUDGEMENTS; DECISIONS: WE THOUGHT WE WERE APCHING THE CORRECT ARPT. WE JUDGED THAT IT WAS THE CORRECT ARPT BY AVAILABLE INFO SUCH AS RWY CONFIGN; DME. FACTORS AFFECTING HUMAN PERFORMANCE: FO HAD BEEN TO THE ARPT FAIRLY RECENTLY AND BELIEVED IT WAS THE RIGHT ARPT. BECAUSE THE FO HAD BEEN INTO ARPT RECENTLY; CAPT BELIEVED IT WHEN FO SAID IT WAS THE RIGHT ARPT. SCENARIO STARTED ABOUT 20 MI FROM THE ARPT. AS WE GOT CLOSER; CAPT BROUGHT OUT CONCERNS ABOUT CORRECTNESS OF ARPT BASED ON RAMP LOCATION. FO FELT IT WAS STILL CORRECT ARPT AND STATED SO. CAPT ASKED FO TO CHK ARPT DIAGRAM. FO DID AND STILL FELT IT WAS THE CORRECT ARPT. FO STATED HE FELT THE MIL ACFT WERE FROM THE GUARD UNIT AT GTF. WHEN ON 2 MI FINAL; FO REALIZED IT WAS WRONG ARPT; SAID SO; AND SUGGESTED GAR. IMMEDIATE GAR WAS MADE AS WE ADDED PWR APCH CTL ASKED IF WE WERE LINED UP FOR 21 AT GTF. WE SAID NO. A MISSED APCH CLRNC WAS ISSUED AND AN UNEVENTFUL LNDG WAS MADE AT GTF. ATC FELT THERE WERE NO PROBLEMS BECAUSE THEY OWNED ALL AIRSPACE AT GTF AND MALMSTROM AFB. THERE WAS NO MIL TFC AT THE TIME. HAD THERE BEEN; THE INCIDENT PROBABLY WOULD NOT HAVE OCCURRED BECAUSE APCH CTL WOULD HAVE MANEUVERED US AROUND. I THINK THE INCIDENT OCCURRED BECAUSE OF MINDSET BY FO AND SLOWNESS OF CAPT TO REACT WHEN QUESTIONS AROSE.