Hey keithj202,
I did the following: check the "use the mouse" option in the game's properties. When in game, for Player 1's button 1, press ENTER to change and press ESC to clear any current assignments. Press enter again and pull your trigger (making sure not to move your gun to register any X/Y axes--I pulled the trigger while resting the gun on the glass). If it registers an axis, repeat the above. It should one have a single assignment (I think something like MOUSE B0).
On a simliar subject: If you're using only one Act Labs gun, as I am, I experienced an issue where both the blue and red (P2) crosshairs moved together to the same spot. I figured out how to remedy that issue as well.
Good luck!