A CARGO FLC OVERSHOOTS THEIR ASSIGNED ALT IN DSCNT WHEN THE CAPT MISUNDERSTANDS THE CLRNC NNW OF SAV; GA.

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DURING ENRTE PHASE TO THE SSI VOR; CTR RERTED US TO OVER THE SAV VOR TO AVOID MIL AIRSPACE. WE NORMALLY DO NOT GET A CHANGE OF COURSE. ENRTE TO SAV; WE WERE GIVEN A DSCNT CLRNC. NORMALLY; ENRTE TO THE SSI VOR; WE EXPECT A DSCNT CLRNC DOWN TO FL240. A DSCNT TO FL240 IS WHAT I BELIEVE I HEARD AND ACCEPTED AND READ BACK. AS I WAS DSNDING THROUGH FL295; THE CTLR SAID THAT I SHOULD BE AT FL310 AND I NEED TO RETURN IMMEDIATELY; WHICH I DID. I STATED TO THE CTLR THAT I THOUGHT THAT I HAD READ BACK FL240. HE STATED THAT I WAS ONLY CLRED TO FL310. I DIDN'T ARGUE WITH HIM. I DID NOT HEAR HIM SAY ANYTHING TO ANY OTHER ACFT SO I DON'T BELIEVE THERE WAS A CONFLICT. I HAD BEEN ON DUTY FOR 10 HRS AND THE SUN WAS JUST PEEKING ABOVE THE HANGAR; SO I WAS A LITTLE TIRED WHEN THIS OCCURRED. THE SIC WAS DOING PAPERWORK WHEN THIS OCCURRED. THE SIC IS ALSO A QUALIFIED PIC. IT IS COMMON PRACTICE FOR THE PF TO FLY AND ANSWER THE RADIOS ENRTE WHILE THE PNF CATCHES UP ON PAPERWORK; SO HE DIDN'T HEAR THE SIT. IF I READ THE CLRNC BACK RIGHT (FL310) THEN I HAD A MENTAL LAPSE IN ASSUMING IT WAS FL240. I WILL CORRECT THIS BY MAKING SURE THE PNF IS ALSO IN THE LOOP. IF I READ BACK FL240 THEN BOTH MYSELF AND THE CTLR ARE AT FAULT -- ME FOR READING IT BACK WRONG; AND HIM FOR NOT CATCHING AND CORRECTING IT.