C172 PRIVATE PILOT AND CFI DEPARTING 2W6 AT NIGHT HAVE HELICOPTER AT MIDFIELD INTRUDE UPON THEIR RUNWAY WHILE DEPARTING.

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CONDITIONS WERE NIGHT; VFR AT 2W6 (ST MARY'S COUNTY REGIONAL ARPT) AND WE (COPLT IS 700 HR CFI) WERE PRACTICING NIGHT TKOF AND LNDGS ON RWY 29. WHILE TAXIING BACK FOR ANOTHER TKOF; WE NOTICED A GOV HELI WITH BEACON ON AND PRE-FLTING IN THE RAMP AREA. UPON REACHING THE END OF THE TXWY; WE ANNOUNCED OUR INTENTIONS TO BACK TAXI THE REMAINING DISTANCE TO THE HOLD SHORT LINE OF RWY 29. AFTER TURNING PERPENDICULAR TO RWY 29 (AND REMAINING BEHIND THE HOLD SHORT LINE); I ANNOUNCED OUR DEP INTENTIONS. AS I CROSSED THE HOLD SHORT LINE WE BOTH HEARD A GARBLED XMISSION FROM THE HELI AND IMMEDIATELY STOPPED JUST BEFORE THE NUMBERS AND ASKED THE HELI TO REPEAT THEIR LAST XMISSION. THE NEXT XMISSION WAS SLIGHTLY CLRER AND WE HEARD THE HELI PLT XMIT 'WE'LL DEPART AFTER YOU.' I THEN REPEATED WHAT THE HELI PLT HAD SAID AND THAT WE WERE 'ON THE ROLL DEPARTING RWY 29.' AT ABOUT 50 FT AGL WE NOTICED THE HELI WAS AT A MIDFIELD TXWY WITH SPINNING ROTORS INTRUDING PAST THE HOLD SHORT LINE. I MANEUVERED TO THE R (N) TO ENSURE CLRNC. AS WE PASSED OVER THE HELI; WE HEARD ANOTHER GARBLED XMISSION THAT SOUNDED SOMETHING LIKE 'GUESS YOU GUYS DIDN'T HEAR US.' SUBSEQUENT XMISSIONS FROM THE HELI (AFTER AIRBORNE) TO ATC WERE LOUD AND CLR. WHILE AT THE APCH END OF RWY 29 WE COULD SEE THE HELI'S LIGHTS AND BEACON; BUT COULD NOT READILY IDENT IF IT WAS BEHIND THE HOLD SHORT LINE AT MIDFIELD (THERE IS A SLIGHT HILL THAT PREVENTS A GOOD VIEW AT NIGHT).