C650 DEP FROM SHD BEGINS CLB TO INCORRECT ALT.

Narrative

WE WERE DEPARTING SHD ON JUN/TUE/03 AT XA10 LCL TIME IN IMC CONDITIONS. WE OBTAINED CLRNC AND RELEASE WHILE ON THE GND AT SHD WITH POTOMAC APCH. WE UNDERSTOOD THE CTLR TO CLR US TO 6000 FT (THE USUAL ASSIGNED ALT; WE FLY INTO THIS UNCTLED ARPT FREQUENTLY); AND IN OUR READBACK WE VERIFIED 6000 FT. SINCE THERE IS NO RCO ON THE FIELD SOMETIMES COMS AREN'T CRYSTAL CLR AND THE CTLR ASKED US TO VERIFY OUR SQUAWK WHICH WE HAD UNDERSTOOD CORRECTLY. HE NEVER ASKED US TO VERIFY OUR ALT READBACK. WE DEPARTED RWY 23 AT SHD AND CALLED POTOMAC APCH OUT OF 5000 FT AT WHICH POINT THEY CAN RADAR IDENT US. POTOMAC APCH CLRED US TO 5000 FT. AT THIS POINT WE WERE AT 5500 FT AND POTOMAC ADVISED US OF TFC AT 6000 FT AND 5 MI. WE IMMEDIATELY RETURNED TO 5000 FT. THE CTLR INDICATED THAT 5000 FT HAD BEEN OUR ORIGINAL CLRNC LIMIT. AT NO POINT DID WE RECEIVE A TCASII ALERT. IT IS THE OPINION OF THE FLT CREW THAT AN RCO ON A FIELD WITH THE AMOUNT OF IFR TFC THAT SHD HAS WOULD DO MUCH TO PREVENT MISCOM BTWN PLTS AND CTLRS.