When shooting irons, I usually shoot with a hooded rear sight. When it is bottomed out, the elevation drum will no longer spin. This is also the case for another USGI sight I own. However the one on my new build is different. When I drop the aperture all the way, the elevation knob will keep spinning even though the aperture itself is not moving. If the go the other way, it will rise like normal. Is this normal? Or not supposed to happen?