Sorry my English is not good ,but i'm here is just for help.
I have download this program:AVA fast gun,
its made just for you so your shooting with gun can be as fast as shooting with m134 (yes, there's no gun in game ;D)
but the hotkeys is X
(HOLD X TO SHOOT WHEN YOU RELASE IT IT WILL STOP SHOOTING)
i wonder if anybody know this program(actually it's very old program)
but it works very well ,i just want it to change the "hotkey=X" to 'hotkey=leftmouse"