FLC DSNDED 300 FT THROUGH THEIR ASSIGNED ALT.

Narrative

WE WERE IN OUR DSCNT FROM ALT ON OUR TRIP FROM CGF TO MDW. ORD APCH HAD CLRED US TO DSND TO 7000 FT. THE ACFT WAS ON AUTOPLT IN THE DSCNT. THE PNF WAS OFF THE FREQ OBTAINING THE CURRENT ATIS INFO. UPON APCH IN 7000 FT; APCH CLRED US TO 6000 FT. I ACKNOWLEDGED THE CLRNC TO 6000 FT AND SET THE NEW ALT IN THE ALT ALERTER. AFTER SETTING IN THE NEW ALT AND CONTINUING THE DSCNT; I TOOK A MOMENT TO STUDY THE APCH CHART. AS I WAS DOING THAT; THE PLT WHO HAD BEEN GETTING THE ATIS SAID; HEY; AREN'T WE CLRED TO 6000 FT. I LOOKED UP; AND WE WERE PASSING 5700 FT. THE COPLT HAD GRABBED THE CTLS TO STOP THE DSCNT. I TOOK THE CTLS AND ASSURED THAT THE AUTOPLT WAS OFF; AND INITIATED A CLB BACK UP TO 6000 FT. AS WE WERE CLBING BACK TO 6000 FT; APCH TOLD US THAT OUR ASSIGNED ALT WAS 6000 FT AND ASKED US WHAT OUR ALT WAS. BY THAT TIME WE WERE LEVELING AT 6000 FT SO WE REPLIED THAT WE WERE LEVEL AT 6000 FT. I BELIEVE THAT THE PROB OCCURRED BECAUSE BOTH PLTS WERE DISTRACTED FROM FLYING THE ACFT DURING THE DSCNT. I SHOULD HAVE BEEN MONITORING THE INSTS EVEN THOUGH THE ACFT WAS ON AUTOPLT. I DON'T KNOW WHY THE AUTOPLT AND FLT DIRECTOR DIDN'T CAPTURE THE ALT; BUT THE INCIDENT WOULD HAVE BEEN PREVENTED IF ONE PLT WAS MONITORING THE INSTS AT ALL TIMES.