Turret Continuing to Max Rotation - Following Mouse Cursor

I have a turret in 3d space that follows the mouse cursor - the base rotates on the x axis, turret rotates on the y.

When the mouse goes past a pre-defined max-angle, the turret stops rotating. I can't figure out a way for it to continue rotating up to the max angle and then stop.

This is what I'm using to rotate the turret:

barrelStart is a quaternion indicating the original rotation of the barrel

barrel is a quaternion indicating the rotation of the barrel to look at the mouse

 if (Quaternion.Angle(barrelStart, barrel) <= maxGunAngle)
                 gun.localRotation = Quaternion.RotateTowards(gun.localRotation, barrel, gunDegreesPerSecond * Time.deltaTime);

Is there a better way to clamp the rotation beyond the If condition I'm using? Or is there a way to calculate what the rotation should be to get to the max angle?

Hope this is clear.. Pretty new to unity. Thanks in advance.