NMAC OCCURS BTWN A GA ACFT AND A DIAMOND DA40 WHEN THERE IS A RWY ASSIGNMENT MIX-UP FOR SEVERAL ACFT ON APCH TO BWI; MD.

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I CONTACTED BWI APCH FOR LNDG APPROX 20 NM W OF BWI. THE FREQ WAS EXTREMELY CONGESTED AND IT TOOK SEVERAL CALLS BEFORE CONTACT WAS MADE. I WAS GIVEN VECTORS N OF BWI IN ORDER TO LINE UP FOR LNDG. THERE WAS SOME PROB ON THE CTLR'S PART UNDERSTANDING OUR TYPE AND CALL SIGN. AFTER ROUTING ME N OF THE ARPT; THEY VECTORED ME FOR LNDG; ALTHOUGH I DID NOT NOTE ANY RWY ASSIGNMENT OR MENTION TO 'EXPECT' A SPECIFIC RWY. THEY CALLED MY 'BASE' BUT WITHOUT ASSIGNING A RWY FOR 3 CLOSELY ALIGNED RWYS. ANOTHER ACFT WAS ON A PRACTICE APCH TO RWY 33R. THE TFC WAS CALLED AND I SEARCHED AND I BELIEVE WE IDENTED AND CALLED THE ACFT 'IN SIGHT' (WITH THE WORKLOAD THE EXACT TIMING OF THIS IS UNCERTAIN IN MY MEMORY). I WAS TOLD I WOULD PRECEDE THE ACFT FOR LNDG AND TO KEEP MY SPD UP AND WAS VECTORED FOR A BASE. I STILL HAD NOT; TO MY BELIEF; RECEIVED A RWY ASSIGNMENT OR CLRNC FOR LNDG. I WAS THEN HANDED OFF TO THE TWR AND CHANGED TO THE TWR FREQ. I WAS UNABLE TO CONTACT THE TWR IN A TIMELY WAY BECAUSE THE TWR WAS CHATTING TO ANOTHER ACFT ABOUT A PARTY THEY HAD RECENTLY HAD. WE HAD STILL NOT RECEIVED A RWY ASSIGNMENT OR CLRNC TO LAND. DURING THIS TIME; I WAS FOCUSING ON WHICH RWY TO LINE UP ON IN THE DUSK. I BELIEVE I NOTED THE OTHER ACFT ON APCH AGAINST THE GND LIGHTS. DURING THIS TIME; I WAS ATTEMPTING TO LINE UP WITH THE ARPT; CONFIGURE THE ACFT FOR LNDG; AND TRYING TO GET A BREAK IN THE RADIO CHAT ABOUT THE PARTY. I TURNED AND SAW THE OTHER ACFT RAPIDLY APCHING FROM MY L AND WE PASSED EXTREMELY CLOSE. I FINALLY WAS ABLE TO CONTACT THE TWR AND ASK FOR CLRNC TO LAND. IT TOOK SEVERAL CALLS TO RECEIVE THE CLRNC WHICH INCLUDED A RWY. BY THAT TIME; I WAS UNABLE TO LAND DUE TO ALT AND ALIGNMENT AND OPTED TO GO AROUND. I BELIEVE THE FOLLOWING FACTS WERE SIGNIFICANT CONTRIBUTIONS TO THE NMAC: POOR SPACING OF THE ACFT ON VECTORS FOR ARR. THE CONGESTION ON THE FREQ MADE WORSE BY A LONG DISCUSSION BY THE TWR OF THE PARTY WITH AN ACFT. FAILURE TO MAINTAIN PROPER VISUAL SEPARATION BY BOTH VFR ACFT DUE TO MISJUDGING OF DISTANCE OR POSSIBLE MISIDENT OF THE TFC AGAINST GND LIGHTS AT DUSK.