AN ACR MLG HAD AN NMAC WITH AN MLT IN THE ONT ARSA ON APCH.

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CLRED DOWN TO 5200 FT. COPLT HAND FLYING. I WAS INSIDE THE COCKPIT MOMENTARILY TO PRESS PA BUTTON FOR 'PREPARE FOR LNDG' PA. COPLT DECLARED THAT A MLT WAS COMING RIGHT AT US. HE BECAME CONCERNED ABOUT WHICH WAY TO GO (TURN) TO AVOID HITTING SAME. I CHKED ALTIMETER WHICH SHOWED WE WERE 200-300 FT BELOW 5200. THAT IS ON THIRD ALTIMETER. CAPT AND FO ALTIMETER ON QFE. THIRD ALTIMETER USUALLY SHOWS HIGH; SLIGHTLY. SO WE MIGHT HAVE BEEN AT 4900 MSL. GOT AN RA. STARTED CLB -- CLR OF CONFLICT. TCASII PERFORMED WELL. ATC DIDN'T POINT OUT MLT AS TFC OR STATE THAT HE WAS REASON WE WERE CLRED TO 5200 FT INSTEAD OF LOWER. MY GUESS IS THAT MLT WAS BEING HANDLED BY NORTON AFB AND WAS RESTR TO SOME LOWER ALT AND WOULD NOT HAVE CLBED THROUGH OUR ASSIGNED ALT EVEN THOUGH IT APPEARED TO BE CLBING. ATC QUESTIONED OUR ALT DURING EVENT; BEFORE RA. COPLT DISCONNECTED AUTOPLT WITHOUT; ME BEING AWARE OF IT. 'AUTOPLT OFF' IS MANDATORY CALL. I'M NOT SURE IF IT WAS MADE OR NOT. IT IS NOT EASILY DETERMINED WHICH WAY TO GO TO GET OUT OF THE WAY OF A BIG JET FILLING YOUR WINDSCREEN. HAPPY ABOUT TCASII.