Sheild isn't really ment for optics and there isn't a need for them on this type of gun. However, they are mounted by milling out a portion of the slide in front of the rear sights. The optic will sit in that cut out portion. Typically this will mean that you'll need to change your regular sights because even though the optic sits lower it's base is still about on the same level as the sights.
Here is an example
You can also buy an adapter that will fit into the cut out of your rear sight but that means you won't have a rear sight.
The Sheild is ment to be an easily concealed, small and light gun. Where as the optic is ment more for full size guns where distance and speed is important. Many of these are competition guns but it can be used on full size and compact carry guns. The Sheild is a sub compact ment for close distances although they can and will shoot center mass at 25 yards.
Also not all optics are created equal
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