WX BRIEFING PRIOR TO DEP STATED THAT BRAKING ACTION WAS FAIR. RPTR LANDED AND DUE TO SLIPPERY RWY RAN OFF THE END. BRAKING ACTION HAD JUST BEEN CHKED AND WAS STILL 'FAIR.' RPTR IS TRYING TO SAY THE BRAKING RPT WAS NOT ACCURATE; BUT DOCUMENTATION DOESN'T SUPPORT THAT ALLEGATION.

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THE RPT STATED THAT THE RWYS WERE IN 'FAIR' BRAKING ACTION CONDITION. IN ADDITION; IT WAS RPTED THAT THE TEMP WAS 26 DEGS F WITH LIGHT SNOW AND 1.25 MI VISIBILITY. UPON APCH TO JACKSON HOLE; MR X AND MR Y WERE DIRECTED TO LAND ON RWY 18. THEY WERE NOT ADVISED OF ANY BRAKING PROBS ON THAT RWY. MR Y CONFIRMED THAT THE RWY WAS IN SIGHT AND THAT THE ACFT WAS ON THE PROPER GS (ON PAPI). MR X TOOK THE ACFT TO VISUAL OPS AND MR Y TRANSITIONED INSIDE. MR Y CONTINUED TO CALL SPDS; SINK RATE; AND ON PROFILE. UPON XING THE THRESHOLD; SPD WAS MAINTAINED AT VREF. ALL ICE PROTECTION SYS WERE ON AT THIS TIME. IN THE LNDG FLARE; MR Y CONFIRMED THE ACFT SPD AT MINUS 3 KTS (102 NM PER HR). PLEASE NOTE THAT THE SPD OF THE ACFT WAS SLIGHTLY BELOW VREF AT LNDG. WHEN THE ACFT TOUCHED DOWN; AT THE 600 FT MARK; THE SPD BRAKES WERE EXTENDED AND MAX BRAKING WAS APPLIED. THE MU300 IS NOT EQUIPPED WITH THRUST-REVERSERS; THEREFORE; ONLY WHEEL BRAKES WERE APPLIED. ALTHOUGH HVY BRAKING CONTINUED THROUGHOUT THE LNDG ROLL; THE ACFT COULD NOT BE STOPPED AND CONTINUED OFF THE S END OF RWY 18 FOR APPROX 232 FT BEYOND THE RWY. THERE WAS NO INJURY TO THE PAX OR CREW; AND THERE WAS NO VISIBLE DAMAGE TO THE ACFT. ACCORDING TO THE JACKSON HOLE ARPT INCIDENT RPT; THE RELEVANT FRICTION MEASURE (RPTED IN MU) WAS STATED AS .51 MU. 20 MINS LATER; AT THE TIME OF THE INCIDENT; A FRICTION MEASUREMENT OF .38 MU WAS RPTED BY ARPT PERSONNEL. ACCORDING TO ARPT DOCUMENTS; THIS WOULD HAVE RESULTED IN A DOWNGRADE OF CONDITIONS FROM 'GOOD' TO 'FAIR.' SUBSEQUENT TO THE INCIDENT; MR X AND MR Y HAVE LEARNED THAT THERE HAVE BEEN NUMEROUS INCIDENTS IN WHICH ACFT HAVE LEFT THE RWY AT JACKSON HOLE DURING THE PAST 4 YRS. MR X AND MR Y ARE TROUBLED BY THE FACT THAT THEY WERE NOT PROVIDED WITH ACCURATE INFO REGARDING THE STATUS OF THE RWY. HAD THE APPROPRIATE BRAKING ACTION CONDITIONS BEEN RPTED; MR X AND MR Y WOULD NOT HAVE ATTEMPTED TO LAND AT JACKSON HOLE.