BE300 KINGAIR FLT CREW ENCOUNTERS WX DURING DEP FROM CHA AND DEVIATES FROM ATC ASSIGNED HDG.

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BE30 (KING AIR 300) CLRED FOR TKOF ON RWY 2 AT CHA. OUR CLRNC WAS TO FLY THE CHA5.CHA DEP; THEN AS FILED. EARLIER ON THAT DAY A COLD FRONT HAD PASSED; WHICH CREATED A STRONG LINE OF VERY INTENSE TSTMS; THEREFORE WE HAD DELAYED OUR DEP IN ORDER TO SEE IF THE LINE WOULD BREAK-UP; BECAUSE WE WOULD HAVE TO FLY THROUGH IT AT SOME POINT. WE DECIDED TO LAUNCH SINCE THERE WERE SEVERAL LARGE BREAKS IN THE LINE WHICH WE FELT WE COULD FLY THROUGH TO GET TO OUR DEST; GMU. OUR CLRNC WOULD ALLOW US TO INTERCEPT V54; AND THAT AIRWAY RAN RIGHT THROUGH ONE OF THE BREAKS IN THE LINE OF TSTMS; WHICH IS ONE OF THE REASONS WE DECIDED TO TAKE OFF AT THAT TIME. ON THIS LEG I WAS ACTING AS PF (L SEAT) AND OUR COMPANY CHIEF PLT WAS ACTING AS PNF AND RADIO OPERATOR. MY NAME WAS ON THE IFR FLT PLAN AS PIC. WE LIFTED OFF AND WERE HANDED OVER TO CHATTANOOGA DEP. THEY AMENDED OUR CLBOUT INSTRUCTIONS. WE WERE NOW INSTRUCTED TO TURN TO A NORTHERN HDG AND MAINTAIN 3000 FT; WHICH WE DID. THIS NEW ASSIGNED HDG WOULD RUN US STRAIGHT INTO HVY PRECIPITATION; AND LEVEL 2 OR 3 EMBEDDED TSTMS ACCORDING TO OUR ONBOARD RADAR; JUST 5 MI OFF OUR NOSE. THE PNF INFORMED DEP OF THIS FACT; AND WE WOULD NEED A 90 DEG TURN...NOW. CHATTANOOGA DEP CAME BACK WITH; 'TURN L TO A HDG OF 270;' WHICH WE LOOKED AT WITHOUT TURNING THE ACFT AND RESPONDED WITH; 'WE CANNOT ACCEPT THAT HDG DUE TO EXTREME WX IN THAT DIRECTION.' DEP CAME BACK IN A VERY TERSE MANNER WITH; 'THEN TURN R TO A HDG OF 050 (DEGS) AND CLB AND MAINTAIN 6000 FT.' THAT NEW HDG AND CLB INSTRUCTIONS DID NOT HELP OUR SITUATION. THE PNF; AGAIN WHO IS OUR COMPANY CHIEF PLT TOLD ME TO KEEP THE TURN GOING PAST 050 DEGS AND DISREGARD ATC'S INSTRUCTIONS; I ROLLED OUT ON A HDG OF 090 (DEGS) WHICH WOULD ALLOW US TO CLB AND PENETRATE LIGHT TO MODERATE PRECIPITATION AND CUMULUS BUILDUPS (IMC). WE STARTED TO ENCOUNTER 'HVY'-MODERATE TURB; HVY RAIN AND HAIL. I SLOWED THE ACFT TO VA SPD. I WAS HAND-FLYING THE ACFT AND AT TIMES USING FULL CTL INPUTS TO KEEP THE ACFT IN A LEVEL; CLBING ATTITUDE. DEP QUERIED US ABOUT OUR DIRECTION OF FLT AND OUR RESPONSE WAS WE NEEDED THIS HDG DUE TO WX. THE RESPONSE FROM CHATTANOOGA DEP WAS; 'I NEED YOU TO FLY 050' (DEGS). THE PNF REPLIED WITH; 'SIR I UNDERSTAND THAT; BUT WE CANNOT ACCEPT THAT HDG DUE TO WX.' AT THIS POINT WE ENTERED AN AREA OF EVEN HEAVIER PRECIPITATION AND SEVERE TURB. THIS IS WHAT CONCERNS ME THE MOST; 1) FLYING A HDG NOT ASSIGNED BY ATC. (WE DID NOT HAVE AN OPTION; AND I UNDERSTAND ATC NEEDS TO HAVE MINIMUM SEPARATION BTWN ACFT.) 2) THE TERSE COM BTWN OUR CHIEF PLT AND THEIR RESPONSE. 3) LACK OF CRM BTWN THE PF AND PNF. THE CTLR ALLOWED US TO STAY ON THE 090 DEG HDG FOR SEVERAL MI; THEN TURN US BACK TO THE 050 DEG HDG AND INSTRUCTED US TO CONTINUE OUR CLB TO 10000 FT. WE COULD ACCEPT THAT HDG THEN AND DID. UPON REACHING 10000 FT WE WERE ON TOP OF MOST OF THE BUILDUPS IN THE AREA. THEN WE WERE TURNED OVER TO ZTL AND THE FLT CONTINUED WITHOUT INCIDENT.