When individual parts from complex rear-sight assemblies go missing, they can be hard to replace because of proprietary features such as thread specifications.

Attaching an AR-15 pistol upper assembly with a barrel shorter than 16" to a rifle lower is also considered an SBR and will get you in trouble ... Rear Sight: UTG Detachable A2 Rear Sight Carry ...

The Co-witness Notch/Peep AR-15 rear sight from Weapon Solutions is designed to prevent the top of the peep from covering up red dot-type optics during co-witness sighting with the AR-15 rifle.