COMMUTER ACFT MISINTERPRETS CLB CLRNC. CLBING TO 6000 FT; CLRNC TO 4000 FT.

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ON A IFR FLT FROM RALEIGH TO CHARLOTTE; NC; AND AFTER HAVING DEPARTED RALEIGH AT 50 Z; RALEIGH DEP ORIGINALLY CLRED US TO 2000 FT. A FEW MINS LATER THEY CLRED US TO LIBERTY VOR AND WITH AN ALT CLRNC TO WHAT WE THOUGHT WAS 6000 FT. 6000 FT WAS OUR FLT PLAN ENRTE ALT AND ALSO THE ALT GIVEN TO US WITH OUR CLRNC FROM RALEIGH CLRNC. APPROX 10 MI W OF RALEIGH AS WE WERE CLBING THROUGH 5000-6000 FT. RALEIGH DEP ASKED US TO STATE OUR ALT; I INFORMED HIM WE WERE PASSING THROUGH 5000-6000 FT. HE SAID THAT WE WERE ONLY CLRED TO 4000 FT; BUT TO MAINTAIN 5000 FT AND NO PROB. BOTH MYSELF AND THE FO EACH HAD 6000 FT PLUGGED INTO OUR RESPECTIVE ALT SELECTORS. BOTH OF US THOUGHT THAT RALEIGH DEP HAD STATED 6000 FT FOR OUR ASSIGNED ALT. CONTRIBUTING FACTORS THAT MAY HAVE LEAD US TO UNDERSTAND INCORRECTLY AN ASSIGNED ALT INCLUDE A 12-HR 10 MIN DUTY DAY BEFORE FOLLOWED BY A REDUCED REST OF 8 HRS 10 MINS. THIS POSSIBLE ALTDEV OCCURRED ON THE SECOND LEG/FLT OF DAY 2 OF A 2 DAY TRIP. A CORRECTIVE ACTION WOULD BE FOR THE FAA TO CHANGE DUTY REGS WHEREBY AIRLINES COULD NOT CONSTANTLY SCHEDULE ON A REG BASIS. REDUCED REST OVERNIGHT TRIPS. ANOTHER CORRECTIVE ACTION WOULD BE TO TAKE THE FAA OUT OF THE AIRLINE'S POCKETS.