CPR MU300 DIAMOND 1-1A(MU3); ROLLED OFF END OF RWY DURING LNDG FROM AN ILS APCH ON A WET RWY. HYDROPLANING IS SUSPECTED.

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WE WERE FLYING COMPANY MEMBERS OF A COMPANY THAT OWNS HALF OF OUR COMPANY/FLT DEPT TO JONESBORO; AR. ZME HAD CLRED US TO FLY THE ILS RWY 23 TO JONESBORO MUNICIPAL ARPT. JONESBORO FSS/RADIO HAD GIVEN US THE FOLLOWING WX: CEILING 500 FT OVCST; VISIBILITY 2 MI WITH LIGHT DRIZZLE AND MIST; TEMP 22 DEGS C; DEWPOINT 21 DEGS C; WIND 160 DEGS AT 9 KTS; ALTIMETER 29.97. RWY 23 IS 5599 FT LONG AND IS STATED TO HAVE A 'POROUS FRICTION COURSE OVERLAY' SURFACE. THE WINDS GAVE US A SLIGHT HEADWIND COMPONENT OF 3 OR 4 KTS; AND SLIGHT XWIND (5 OR 6 KTS) FROM THE L. OUR CALCULATED LNDG WT OF 13500 LBS GAVE US A VREF SPD OF 110 KIAS AND A REQUIRED LNDG DISTANCE OF 2940 FT. THE CAPT (PF) FLEW THE ILS RWY 23 APCH AS PUBLISHED AFTER BEING RADAR VECTORED TO INTERCEPT THE FINAL APCH COURSE. WE HAD BEEN REQUESTED TO KEEP OUR SPD UP BY CTR TO THE BEST OF MY RECOLLECTION. INTERCEPTION OF BOTH THE LOC AND GS WENT NORMALLY AND THE BEFORE LNDG CHKLIST WAS COMPLETED WITH LNDG GEAR DOWN. THE CAPT (PF) HAD TO REDUCE THRUST TO IDLE AS WE DSNDED ON THE GS DUE TO THE SLIGHTLY HIGHER SPD WE HAD BEEN REQUESTED TO MAINTAIN. THE ACFT SLOWED CONTINUOUSLY AND I RECALL CALLING OUT EITHER VREF +30 (140 KIAS) OR VREF +40 (150 KIAS) AS WE INTERCEPTED THE GS. THE ACFT HAD SLOWED TO APPROX VREF +10 (120 KIAS) AS WE APCHED DECISION HT OF 461 FT MSL; WHICH HAD US APPROX 40 FT BELOW THE CEILING AND THE RWY IN CLR SIGHT. THE ACFT TOUCHED DOWN ONTO THE RWY SOMEWHAT BEFORE THE POINT (1000 FT) THAT THE GS WOULD HAVE TAKEN US TO; IN ORDER TO UTILIZE AS MUCH RWY AS POSSIBLE. TOUCHDOWN/LNDG SPD WAS SOMEWHERE BTWN VREF (110 KIAS) AND VREF +10 (120 KIAS) AS BEST AS I RECALL. I EXTENDED THE SPD BRAKES IMMEDIATELY AFTER LNDG AND ANNOUNCED MY ACTION. I COULD SENSE/FEEL SOME BRAKING ACTION AS THE CAPT APPLIED BRAKES SOMEWHAT SMOOTHLY ON THE SLIGHTLY WET RWY AND THE LNDG ROLLOUT SEEMED NORMAL AT FIRST. HOWEVER; I COULD SENSE THAT WE WERE NOT SLOWING AS WE NORMALLY WOULD AND I COULD NOT FEEL ANY ANTI-SKID CYCLING AND/OR NORMAL BRAKE 'JERKING' FELT AT TIMES. THE CAPT FELT THE ACFT SLIDE SOMEWHAT AS THE ACFT WENT OVER SOME PAINTED NUMBERS/LINES ON THE RWY. HE LET OFF THE BRAKES AND REAPPLIED THEM. WE HAD NO INDICATION OF ANTI-SKID FAILURE (ANNUNCIATOR LIGHT NOT ON) AND AT NO TIME DID IT FEEL AS IF WE WERE SKIDDING. THE ACFT CONTINUED STRAIGHT ON THE RWY. THE ACFT HAD BEEN SLOWED TO APPROX 10-20 KTS BY THE TIME WE REACHED THE LAST 200 FT OF THE RWY; AND IT DID NOT SEEM THAT THE ACFT COULD BE STOPPED ON THE REMAINING RWY WITHOUT EXTREME BRAKING OR POSSIBLY A VERY HARD; DANGEROUS TURN. I COULD CLRLY SEE THAT THE GND BEYOND THE END OF THE RWY WAS VERY LEVEL AND THE GRASS WAS CUT SHORT. I SUGGESTED THE OPTION OF KEEPING THE ACFT ROLLING AS WE WERE AT THE END OF RWY 23; AND EITHER ROLLING AHEAD TO RWY 14/32 AHEAD OR TURNING AROUND. THE CAPT CHOSE TO KEEP THE ACFT ROLLING AS THE SAFEST OPTION; ADDED SOME PWR; AND ROLLED THE ACFT STRAIGHT AHEAD OVER THE LEVEL GRASS TO RWY 14/32. THE ACFT WAS THEN TAXIED OFF RWY 32 ONTO A TXWY TO THE RAMP AFTER THE VERY SMOOTH ROLL ACROSS THE GRASS. THE PATH WAS CLR AND WE DID NOT COME CLOSE TO ANY TAXI LIGHTS OR OTHER OBJECTS OR STRUCTURES. NO INJURIES WERE SUSTAINED (ONLY 1 PAX EVEN NOTICED WHAT HAD HAPPENED) AND NO DAMAGE HAD OCCURRED TO THE ACFT OR ARPT PROPERTY. NO FLAT SPOTS OR UNEVEN WEAR WERE NOTED ON ANY OF THE ACFT TIRES AND SUBSEQUENT INSPECTION/TESTING OF THE BRAKES BY COMPANY MECHS SHOWED NO PROBS.