i think its something like
man fire [m203muzzle,flarered]
but i forget, its ages since ive worked with the fire command, best thing i can suggest is playing as a guy with flares, add this to the units INIT
this addeventhandler ["Fired",{hint format ["this fire [%1,%2]",_this select 2,_this select 3]}]
and it will tell you exactly how to use the fire cammand