Redundancy is often a good thing, especially in sighting equipment.
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.
Referred to as BUIS for short, these sights are becoming increasingly popular as flattop ARs replace the conventional-carry-handle models. Currently, there are a host of models and types from which to choose.
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.
One of the advantages of a nonmagnified optical gun sight, such as an Aimpoint Red Dot or EoTech Holographic Weapons Sight, is the ability to mount it to cowitnesses with your iron sights. Cowitness simply means your optic is mounted on the same plane as your iron sights, allowing you to aim through your optic when using your iron sights.
Nonmagnified optical sights can be mounted to allow the iron sights to cowitness through the center axis of the optic's field of view (FOV) or offset from the center axis. When mounted in an offset manner, the iron sights usually look through the lower portion of the optic's FOV. How you prefer to mount your optic to cowitness is purely subjective. I prefer my iron sights to cowitness through the lower portion of my optic's FOV; others prefer the center. Neither is right or wrong, just what you prefer.
Both types of BUIS have their advantages and disadvantages. Fixed models tend to be simpler and more robust. Plus, as they do not need to be deployed prior to use, they are quicker into action. Their primary drawbacks are they take up part of the optic's FOV, and they can snag on things. Flip-up sights fold neatly out of the way when they are not required. This removes them from the FOV of a nonmagnified optic and allows them to nestle beneath a magnified optic. By folding, they also reduce the chance of being caught on something. However, they do require manual deployment before use and add complexity (and possible weakness) to the design.
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.
Next, understand that in a backup or emergency sight, simple is better. Elaborate windage and elevation adjustments on the rear sight are neither necessary nor desirable. Keep in mind that realistically, you are not going to be using them to engage targets 600 meters away while cranking in windage corrections. Due to this, I consider large, easy-to-turn A2-style windage knobs a liability. The chances of them being accidentally turned, and losing your zero, is significantly greater than the likelihood of you ever needing to touch the knob in the field.
Far better is an A1-style dial with a positive lock (such as found on Troy Industries' sight) or a low-profile knob (like used on an A.R.M.S. #40-L). Elevation? Remember, we're talking about emergency use on a high-velocity 5.56. Zeroed at 50 yards the rifle will shoot point of aim at 200 yards and will only require a relatively small amount of holdover at 300 yards. Due to this, I don't feel the need for a separate elevation-adjustment system. A simple dual-aperture L-shaped flip sight is sufficient for my needs.
If I were headed to Afghanistan I might consider a KAC 600-meter sight or a Matech (for peace of mind), but this would be the exception, not the rule.
When choosing aftermarket front-sight bases, there are a couple of things to consider. Does it use a standard AR-style front sight? Will it accept a standard front-sight-adjustment tool? How are the protective ears contoured? These are just a few things to keep in mind.
As most aftermarket sights are crossbolted in place, proper installation is essential. This is due to the firing vibrations of an automatic rifle having a tendency to loosen bolts over time. Unchecked, this can lead to your sight loosening and/or departing for places unknown. To prevent this, make sure you clean the threads and then use a proper amount of Loctite when installing your sights.
My recommendation? Carefully evaluate what your personal needs and desires are, and select a BUIS accordingly. Personally, I've been impressed with the Troy Industries Folding Battle Sight (both front and rear), A.R.M.S. #40-L flip-up rear sight, Precision Reflex and Les Baer's flip-up front sights and LaRue Tactical's fixed rear sight. All of these are good products from good companies.
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