Algodoo

bendTarget = ((a, b)=>{
math.atan2(b(1) - a(1), b(0) - a(0))
}(scene.my.gun.pos, app.mousepos) + 2.5) / 3 * (-1)

equals

v := app.mousepos - scene.my.gun.pos;
bendTarget = -(math.atan2(v(1), v(0)) + 2.5) / 3

Maybe more accurate aiming if you use difference between the laser angle and the (mouse.pos - laser.pos) angle to drive the bicep motor.

For more accurate aiming do this:

laser.onSpawn:
scene.my.laser = entity

Save and reload scene to set scene.my.laser.

Bicep Hinge:
motorTorque := 5.0000000;
bendTarget := NaN;
bend := false;
postStep := (e)=>{
v := app.mousepos - scene.my.laser.pos;
targetAngle := math.atan2(v(1), v(0));
errorAngle := (targetAngle - scene.my.laser.rotation);
motorSpeed = 10.0 * errorAngle
};
motor := true;
bendConstant := NaN;
autoBend := true;

Wow that is awesomely accurate! Thanks