NMAC BTWN A DEP; CLBING VECTORED IFR BE20 OVERTAKING A CLBING VFR C172 THAT HAD DEPARTED FROM THE SAME ARPT; HEF; VA.

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OUR CLRNC TO RIC WAS TO FLY THE ARSENAL ONE DEP TO CSN AND EXPECT OUR NORMAL ROUTING TO RIC. WHILE ON THE GND; TWR GAVE US 2 TARGETS THAT HAD TAKEN OFF FROM RWY 34L. WE DEPARTED RWY 34R FROM HEF AND WERE CLBING THROUGH 800 FT ENRTE TO 2000 FT. WE WERE THEN GIVEN A HEADING OF 270 DEGS AND TOLD TO CHANGE FREQ TO POTOMAC DEP. WHEN WE CHKED IN WITH POTOMAC; THE CTLR GAVE US A TA; 'CESSNA AT 11 O'CLOCK POS; 1700 FT CLBING; 1/2 MI.' THE CTLR'S ADVISORY WAS IN A MATTER-OF-FACT VOICE. HE DID NOT SEEM IN THE LEAST BIT CONCERNED. AT THAT TIME WE WERE LEVELING AT 2000 FT. I SAW THE CESSNA IN A FEW SECONDS OUT THE L VENT WINDOW AS WE WERE PASSING OVER IT. WE PROBABLY PASSED OVER IT BY ABOUT 200 FT. I MADE NO MANEUVERS WITH THE ACFT ON THE CALL; BECAUSE I HAD NO IDEA WHICH WAY TO TURN. OUR ACFT IS NOT EQUIPPED WITH ANY FORM OF TCASII. THE CTLR DID NOT MAKE FURTHER MENTION OF THE TFC; NOR DID WE. I REALIZE THAT PEOPLE MAKE MISTAKES; BUT COORD BTWN MANASSAS TWR AND POTOMAC DEP SHOULD BE ADDRESSED. BECAUSE OF INSTANCES LIKE THIS; WE HOPE TO HAVE ALL OF OUR ACFT EQUIPPED WITH TCASII SOON.