CL65 FLC OVERSHOOT SID ALT ON DEP. PIC QUESTIONS WHY ATC CLRNC DELIVERY DID NOT CATCH INCORRECT READBACK.

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DURING READBACK OF CLRNC TO CLRNC DELIVERY; MY FO CLAIMS HE READ BACK 6000 FT AS THE INITIAL CLRED ALT. THIS WOULD HAVE BEEN A NONSTANDARD ALT AND SHOULD HAVE BEEN CONFIRMED BY ATC. DURING MY DEP BRIEFING; WE WERE INTERRUPTED BY A COMPANY MECH BRINGING SPARE FUSES ONTO THE ACFT. THIS WOULD HAVE BEEN THE APPROX PLACE IN THE BRIEFING WHICH I WOULD HAVE BEEN POINTED TO THE CLRED ALT OF THE SID (NORMALLY 3000 FT). AFTER DEP; WE WERE CLBING TO 6000 FT WHEN WE WERE 'CHKING IN' WITH DULLES DEP CTL. THERE WAS APPROX 20 SECONDS ACKNOWLEDGING OUR CHK-IN. DURING THIS DELAY; WE BEGAN TO CLB OUT OF 3000 FT; WHICH WAS THE STANDARD SID ALT. I HAD AN UNEASY FEELING THAT SOMETHING WAS NOT RIGHT AND BEGAN TO QUESTION THE ALT WITH MY FO. THINGS WERE HAPPENING VERY QUICKLY AND I MADE AN 'EXECUTIVE DECISION' TO CORRECT DOWN TO THE NORMAL SID ALT. BY THE TIME THE ACFT'S INERTIA HAD CHANGED FROM A CLB TO A DSCNT; WE HAD CLBED 500 FT ABOVE THE NORMAL SID ALT. AS WE WERE CORRECTING; ATC FINALLY ACKNOWLEDGED OUR CALL SIGN WITH 'SAY ALT.' WE WERE PRESENTLY LEVELING 3000 FT AT THAT TIME AND STATED THAT FACT. DULLES DEP CTL MADE NO FURTHER INQUIRIES WITH US AND CLRED US UP TO 7000 FT. THE REMAINDER OF THE FLT WAS UNEVENTFUL. THE FLC BELIEVES THE FOLLOWING LED TO THE POSSIBLE ALTDEV: 1) CLRNC DELIVERY SHOULD HAVE CORRECTED THE FO'S READBACK OF INITIAL ALT IF IT HAD BEEN INCORRECT. 2) INTERP OF DEP BRIEFING BY MECH DURING A CRITICAL PART OF THE BRIEFING. 3) AFTER DEP; THERE WAS A SIGNIFICANT DELAY CHKING IN WITH DEP CTL (POSSIBLE CTLR CHANGE?). IN THE FUTURE I PLAN TO DO THE FOLLOWING: 1) BRIEFINGS WILL BE STARTED OVER IF THE CREW IS INTERRUPTED OUTSIDE OF THEIR CTL. 2) DEP BRIEFINGS WILL BE COMPLETED BEFORE ANOTHER TASK (IE; MAINT ITEM; JUMP SEATER; ETC). 3) I PLAN TO BE MORE PROACTIVE VERIFYING ALTS WITH ATC IF CLRED TO ALTS OTHER THAN DEPICTED ON COMMERCIAL PLATES.