GLF2 CONTACTS BE20 DURING CLOSE PROX TAXIING AT MDW.

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WE WERE CLRED TO TAXI TO RWY 31C AT MDW. WE WERE TOLD BY GND WE WERE CLRED WHEN WE COULD GET BY THE KING AIR WHO WAS HOLDING SHORT OF RWY 31L. AS I APCHED THE KING AIR; I STOPPED; SEEING THAT I DIDN'T HAVE THE ROOM TO GET BY. THEN THE KING MOVED FORWARD; SO I ASKED MY COPLT IF WE HAD ROOM ON THE R. HE INFORMED ME THAT WE DID. AT THAT TIME I MOVED FORWARD AND TO THE R TO SEE IF WE HAD ROOM ON THE L TO GET BY THE KING AIR. I LOOKED OVER MY L SHOULDER AND SAW THAT WE STILL WEREN'T GOING TO GET BY. I STEPPED ON THE BRAKES AND AT THAT TIME; I MADE CONTACT WITH THE TAIL CONE OF THE KING AIR. BOTH ACFT TAXIED BACK TO THE FBO WHERE WE FOUND OUT THAT MY ACFT ONLY HAD A COUPLE OF SCRATCHES AND THE KING AIR HAD MINOR DAMAGE TO ITS TAIL; CONE. THERE IS NO EXCUSE FOR THIS HAPPENING; AND I TAKE FULL RESPONSIBILITY FOR THE INCIDENT. I JUST MISJUDGED THE DISTANCE BTWN MY WINGTIP AND THE OTHER ACFT. THIS IS THE WORST THING THAT HAS HAPPENED TO ME IN MY AVIATION CAREER; AND I WILL ALWAYS QUESTION MYSELF ABOUT THIS!