Both front and rear sights are also compatible with any standard AR-15/M4 sights, including any standard height back up iron sights or the detachable carry handle rear sights. This offers the shooter maximum sight options. The new Centurion Arms sights also take up a minimal amount of rail space leaving plenty of room to be used as back up iron sights in conjunction with Aimpoint, EO Tech and most other readily available red dot sights.
A backup sight is intended to be just that--an emergency sight in case of scope failure. A simple, yet rugged design like this Troy Industries is preferred. Without a doubt, the latest generation of optical gunsights offers many advantages over traditional iron sights. Due to this, iron sights are increasingly being relegated from a primary to a backup role. This in turn has led to a virtual explosion of aftermarket AR-15 backup iron sights over the last couple of years.
BUIS refers to any iron sight mounted onto a rifle that is fitted with an optical sight as a primary sighting system. They come in two basic configurations, fixed or flip-up. As their name implies, fixed BUIS are mounted at the proper plane for their employment. This type of BUIS is always ready for use. Flip-up backup sights, on the other hand, rotate out of the optical sight's plane when not required. This type of sight needs to be raised prior to use. It's possible to use fixed BUIS front and rear, flip-up front and rear or to combine the two systems.
Currently, there are a host of BUIS designs available from a wide variety of manufacturers. Some of these sights are very simple while others are fairly complex. When choosing one for your rifle, keep in mind that they call them a backup iron sight for a reason. Your optic sight is intended to be your primary sighting system. The only time you should have to employ your iron sights is if your optic fails. More than likely this means your rifle suffered a drop or blow hard enough to break your scope. So, in my opinion a BUIS should be first and foremost rugged. Look for a design that's well made from strong materials that attaches securely to the rifle.