AN ASTRA CORPORATE PIC'S RPT ON A POSSIBLE NMAC WITH A G2 TURNING FINAL FOR RWY 11 WHILE 3 MI OUT ON ILS FINAL APCH TO RWY 11 AT BED; MA.

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MY ACFT WAS CLRED FOR A RWY 11 ILS; CIRCLE TO RWY 5; THEN CHANGED TO STRAIGHT IN ILS RWY 11. INSIDE LOM WE GOT A TCASII WARNING AND THEN RA AS WE BROKE OUT OF THE CLOUDS. WE SAW A G2 TURNING BASE INTO AND UNDER US ABOUT 500 FT BELOW. BEING VISUAL AND WITH TWR TELLING US TO ENTER RWY 5 PATTERN; WE COMPLETED THE LNDG ON RWY 5 AS ACFT Y ROLLED OUT RWY 11. THERE WAS A BAD LACK OF COORD BTWN BOSTON AND HANSCOM FIELD TWR ALLOWING A VFR G2 TO TURN INTO A CLRED ASTRA ON ILS FINAL. THIS COULD HAVE BEEN AN ACCIDENT VERY EASILY. PLEASE EMPHASIZE COORD BTWN APCH FACILITIES AND TWRS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR; PIC; STATED THAT THEY HAD MADE A CALL TO THE TWR AFTER THE LOM HAD BEEN PASSED; BY ABOUT 1 MI OR SO. ABOUT THAT TIME THEY WERE BREAKING INTO VMC CONDITIONS; SEEING THE G2 ON A R BASE. THEY PERFORMED A GAR EVASIVE ACTION AND THE G2 LANDED RWY 11 AS PLANNED. RPTR BELIEVES THAT THE INCIDENT WAS BASED UPON A LACK OF COORD BTWN A90 APCH CTLR AND THE TWR AT BED. THE G2 WAS ALWAYS THERE ON A TRAINING FLT. IT WAS A MARGINAL VFR DAY. HORIZ MISS DISTANCE WAS ABOUT 1/4 MI OR LESS.