AN IFR C310 PILOT DESCRIBES A VMC NMAC DURING A DESCENT TO HIS DESTINATION. AN ADVANCED GA NAV TRAFFIC ALERT ALLOWED FOR AN AVOIDANCE MANEUVER.

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I HAD BEEN CLRED TO DSND FROM 8000 FT; MY ENRTE IFR ALT FOR OVER 2 HRS; TO 4000 FT AS I NEARED MY DEST. I REDUCED PWR AND STARTED MY DSCNT AT ABOUT 500 FPM. FAIRLY QUICKLY I RECEIVED A 'TFC; TFC' AUDIO ALERT FROM THE SKYWATCH TFC ALERT SYS OPERATING THROUGH MY GARMIN 530. WHEN I LOOKED AT THE 530; THE SMALL TFC SCREEN HAD ACTIVATED AND INDICATED A TARGET AT MY 9 O'CLOCK POS; SLIGHTLY BELOW ME AND IT WAS WELL INSIDE THE 2 MI RING. I ACTIVATED THE LARGE TFC SCREEN AND TRIED TO FIGURE OUT WHAT THE TFC WAS DOING -- XING; OPPOSING; CLBING; DSNDING; OR WHAT. WITHIN SECONDS; I SAW THAT IT WAS MOVING DIRECTLY TOWARD ME AT 9 O'CLOCK POS AND NOW AT MY ALT AND THE TARGET WAS ALMOST TOUCHING MY ACFT SYMBOL ON THE SCREEN. I PUSHED THE NOSE DOWN HARD AND BANKED HARD R. WHEN I LOOKED OVER MY L SHOULDER; I SAW THE TARGET PLANE EXTREMELY CLOSE PASSING DIRECTLY OVERHEAD. I DON'T BELIEVE THE OTHER PLT WAS AWARE OF THE NEAR MISS AS HE NEVER DEVIATED FROM HIS STRAIGHT AND LEVEL FLT. THE WX WAS CLR WITH GOOD VISIBILITY. WHEN I QUERIED ATC IF THEY SAW THAT ACFT THAT HAD JUST PASSED OVER ME; THEY HESITATED AND THEN SAID; YES THEY NOW SEE THE TFC AND IT IS AT 7500 FT. AT THIS TIME I WAS AT 7000 FT AND CONTINUING MY DSCNT. ONE OF THE SHORTCOMINGS I HAVE SEEN WITH THE SKYWATCH IS THAT TFC THAT IS BELOW THE SHADOW OF MY ACFT IS NOT ALWAYS SHOWN UNTIL IT IS QUITE CLOSE -- SOMETIMES LESS THAN 1 MI. AT THE LOW 9 O'CLOCK POS; I BELIEVE MY WING WAS BLANKING OUT THE TARGET'S XPONDER RESPONSE UNTIL HE WAS VERY CLOSE. I FIRMLY BELIEVE THAT WITHOUT THE ALERT THAT THERE WAS A GOOD LIKELIHOOD OF BENT METAL. EXPERIENCE HAS SHOWN ME IN OVER 45 YRS OF FLYING THAT OTHER ACFT ARE AT TIMES EXTREMELY HARD TO SEE -- EVEN IN GOOD WX AT FAIRLY CLOSE RANGE. THE SKYWATCH FREQUENTLY SHOWS ACFT WITHIN 1 MI OR SO AND WITHIN 500 FT THAT ATC MAY OR MAY NOT ALERT ME TO THAT I OR MY PAX ARE UNABLE TO VISUALLY PICK OUT. IT HASN'T STOPPED ME FROM TRYING MY BEST TO WATCH FOR OTHER ACFT -- AS ATC OCCASIONALLY MISSES OR DOES NOT CALL A TARGET THAT MIGHT BE A POTENTIAL CONFLICT.