If they are back-up sights, with all that the designation implies, there's no need to get anything fancier than the Magpul MBUS set. They are a excellent compromise of quality and value.
If you intend to use them for precision, longer ranges or competition work, then there are other, more expensive sights, Troy for example, that lend themselves well to that use.
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Shoulder Drive Nicholson Club
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