The M223XR mount has 20moa built in. Typical 5.56 has 20" drop at 400yds +/-. At 500yds the drop can be 60"+/-.
If you want to shoot past 500yds a 20moa raiser is pretty much need to have, depending on the internal travel of your scope. As the scope nears the end of the vertical travel it looses horizontal travel. Using a raiser keeps the scope closer to the mechanical center at longer ranges.