ACR FLC DSNDED BELOW CLASS B FLOOR WHILE ON VISUAL APCH CLRNC.

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ON JUL/SUN/95 I WAS THE FO ON AN ACR FLT. THIS WAS A CHARTER THAT ORIGINATED OUT OF SARISOTA; FL; STOPPED IN DAYTONA BEACH; THEN CONTINUED ON TO LAS VEGAS; NV. OUR INCIDENT OCCURRED AT 7000 FT MSL; 23 NM E OF LAS ON THE CTRLINE OF RWY 25L LOC. THE WX WAS CLR WITH UNLIMITED VISIBILITY AND CALM WINDS. THE CAPT WAS FLYING THE ACFT AND I WAS PERFORMING THE PNF DUTIES. WE WERE CLRED TO CROSS CROWE INTXN AT 11000 FT. WE HAD NUMEROUS TA'S FROM ATC UP TO THIS POINT. ON INITIAL CONTACT WITH LAS APCH; WE WERE TOLD TO DSND TO 8000 FT. I PLANNED TO MAKE A PA TO THE PAX AND TELL THE FLT ATTENDANTS TO PREPARE FOR LNDG. WE THEN GOT A TA FROM ATC AND WERE TOLD TO EXPEDITE DOWN TO 8000 FT TO AVOID 9500 FT TFC OFF THE NOSE AT 12 NM. I FELT THIS WASN'T A TIME TO BE 'OUT OF THE LOOP' ON THE PA WHEN I SHOULD BE CLRING FOR TFC; AND SUBSEQUENTLY STAYED ON THE ATC FREQ. THIS TFC INITIALLY WAS A TCASII TA THEN RESOLVED TO AN RA. THE CONFLICT ACFT WAS IN SIGHT AT THAT TIME; OUR VERT SPD WAS NOT IN THE RED 'AVOIDANCE AREA' SO WE LEVELED OUT AT 8000 FT AND EVENTUALLY RECEIVED A TCASII 'CLR OF TFC.' ATC THEN CLRED US FOR THE VISUAL APCH WHICH I ACKNOWLEDGED; THEN TOLD THE CAPT THAT I WOULD BE OFF ATC FREQ MAKING A PA. WE WERE AT 27 DME OFF OF LAS ON THE RWY 25L LOC CTRLINE. ONCE I WAS 'OUT OF THE LOOP;' THE CAPT STARTED A VISUAL DSCNT. WHEN MY PA WAS OVER; I RETURNED TO ATC FREQ AND NOTICED WE WERE BELOW THE RWY 25L ILS GS. I IMMEDIATELY TUNED THE LAS VOR AND REALIZED THAT WE WERE BELOW THE CLASS B AIRSPACE WITH ABOUT 3 MI REMAINING BEFORE WE WOULD BE BACK IN IT AT THE 20 NM POINT. I RESET THE ALT SELECTOR WINDOW TO 6500 FT AND TOLD THE CAPT TO STOP HIS DSCNT BECAUSE WE WERE OUT OF THE CLASS B AIRSPACE AND THE FLOOR AT 20 DME WAS AT 6500 FT (AT THIS POINT WE WERE 23 DME). ATC NOTICED OUR DEV AND TOLD US WE WERE BELOW CLASS B AIRSPACE; TO NOT DSND BELOW 6000 FT BECAUSE OF VFR TFC AT 12 O'CLOCK. BUSTED! ATC EVENTUALLY CLRED US AGAIN FOR THE VISUAL. I FELT JUSTIFIED TO VIOLATE THE STERILE COCKPIT BELOW 10000 FT AS A SAFETY OF FLT ITEM TO INFORM THE FLT ATTENDANTS OF LNDG. UNFORTUNATELY; THE MIN I WAS NEEDED MOST TO BACKUP THE CAPT OUT OF THAT 5 HR FLT; I WAS ON THE PA. LESSON LEARNED: START MAKING PA'S AT A HIGHER ALT FARTHER FROM THE AERODROME SO I CAN BE 'IN THE LOOP' WHEN IT GETS BUSY. SUPPLEMENTAL INFO FROM ACN 309388: FLT WAS ON CROWE 6 ARR. WE CALLED THE ARPT IN SIGHT AND WERE CLRED A VISUAL APCH TO RWY 25L. IN THE RECENT PAST; INSTRUCTIONS FROM ATC HAVE RESULTED IN US BEING HIGH ON APCH; NECESSITATING S-TURNS OR EARLY GEAR EXTENSION TO GET DOWN. THIS INFLUENCED MY DECISION TO DSND A BIT EARLIER THAN USUAL. UPON BEING CLRED FOR THE VISUAL; I SWITCHED TO VOR-ILS; THE HSI SHOWED NO GS OR LOC. I BELIEVE THIS HAPPENED AS WE WERE DSNDING NEAR 8000 FT. I CHKED THE FREQ IN THE APCH PLATE. THE FREQ WAS 111.75 WHEREAS THE FMS SHOWED 111.80. I DON'T BELIEVE THIS WOULD HAVE HAPPENED IN ANYTHING BUT CAVU CONDITIONS. UNDER IFR; I WOULD HAVE CHKED THE ILS SOONER. THOUGH NOT TRYING TO LESSEN MY RESPONSIBILITY; A CLRNC SUCH AS 'CLRED VISUAL APCH; CROSS 20 DME AT 8000 FT' MIGHT PRECLUDE THIS IN THE FUTURE. I WILL COORDINATE CLASS B RESTRS AND ARR AND APCH PROCS MORE CLOSELY IN THE FUTURE.