PA28 PLT WAS DUMPED TO A VFR XPONDER CODE WHEN FLYING TO AN ARPT ON THE EDGE OF DC ADIZ AIRSPACE.

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ON APR/TUE/03; DEPARTED CYX APPROX XA40 FOR PRACTICE APCHS AT THV AND DMW VOR RWY 34 VIA EMI. COMPLETED NDB AND MISSED APCH AT THV. HBG APCH ADVISED I CONTACT POTOMAC APCH FOR THE SBOUND LEG ON FREQ 125.52. I DID; AND WAS ASSIGNED CODE (IF MEMORY SERVES). UPON APCHING PROX OF EMI (ABOUT 5 NM N); I WAS INSTRUCTED TO MAINTAIN 4300 FT UNTIL ESTABLISHED; 'CLRED FOR THE APCH; FREQ CHANGE APPROVED; SQUAWK 1200.' I WAS DISSATISFIED WITH MY INBOUND COURSE ENTRY WHICH IS PUBLISHED AS 358 DEGS; SO I DID A COURSE REVERSAL TO TRY AGAIN. THE 178 DEG LEG WAS EXTENDED APPROX 8 NM S OF EMI TO GIVE PLENTY OF TIME TO SET UP INBOUND. I REMAINED AT 3000 FT; CROSSED THE VOR; AND COMPLETED THE APCH TO A CIRCLE-TO-LAND RWY 16. THEN I WAS CALLED ON UNICOM TO LAND AND CALL A PHONE NUMBER. THEY HAD ME CALL POTOMAC TRACON. MR X INFORMED ME I HAD ENTERED THE WASHINGTON ADIZ. AFTER THE CALL; I COULD SEE THAT I HAD PENETRATED THE VEIL (THOUGH BELOW CLASS B FLOOR ALT.) UPON REFLECTION; IT IS MY BELIEF THAT THE CTLR WHO TOLD ME TO SWITCH FROM THE DISCRETE CODE TO 1200 BEFORE I WAS ESTABLISHED INBOUND WAS A CONTRIBUTING FACTOR TO THE ALLEGED INFRACTION.