The Strike Polymer Backup Sights (SPBUS) is a low-profile affordable sighting system that is designed for the AR platform and has bonus built-in sights when flipped down. No time to flip up the sights, you have backup sights on your backup sights. The SPBUS sights are made with lightweight and durable SI Polymer Extreme material with steel mounting tab and insert that can handle any wear and tear you throw at it without any major issues. As long as you've got a Picatinny rail, you will have these SPBUS mounted in minutes. Both sights are capable of flipping down nice and low so you don't have to worry about them being in your way when they're not in use or snag on any gear. The SPBUS does not have an internal spring so you don't accidently deploy the sights when you don't need them. Since the SPBUS sights are slightly taller than standard A2 sights (2.3mm/0.09" taller), the SPBUS is designed and optimized to have both sights paired together and not with other sights. To zero the sights or make any adjustments, the rear sight has an easy-to-use knob for adjusting windage while the front sight elevation is easily and quickly adjusted without tools. The rear SPBUS has dual apertures at 4.70mm(0.19") for longer eye relief, low light and faster target acquisition; and 1.75mm(0.07") for shorter eye relief and greater accuracy. Quickly slide the rear sight up and down to change to the aperture you need.