AR-15 — Mounting Backup “Iron” Sights (BUIS)

BUIS:

BUIS is an acronym for “backup iron sights”. This is nothing more than a catchy label for backup sights that you can use when and if your primary electronic or magnified optic is damaged or fails. “Iron” simply refers to their metal parts to differentiate them from electronic optics, although none of them are made from iron, but rather aluminum, steel, and plastic. Backup sights can simply be a rear sight that sits on a flat-top receiver behind the optic, or it may be a set of both front and rear sights for an ORC (Optic Ready Carbine: a carbine with only rails on top and no front sight base that is ready for customization by the end user).

Most rear backup sights are only capable of windage adjustments. Elevation must be established with adjustments to the front sight. Most backup sights do not have the fine adjustments like those on a carry handle. Zero them as close as possible, but don’t expect perfection. Zero your backup sights for point-of-aim / point-of-impact at 50 yards. Be sure to use Loctite thread locker when mounting your backup sights. I promise that the screws will eventually loosen during shooting and the sights will lose their zero.

If you run some type of aiming device (holographic sight, red-dot sight, or magnified optic) on your rifle then you need to have a backup sight(s) on your rifle. Murphy’s Law dictates that your electronic optics will break or be damage, your scopes will break or fog up, and your batteries will go dead at the worst time. If this happens and you don’t have a backup system for aiming, then that weapon is out of the fight. You soon will be too.

One day, after making a long drive to the range to shoot my M4, I discovered that my Eotech’s batteries were dead. Although I was ashamed for not having an extra set of batteries with my rifle or in my range bag, I was still able to utilize the rifle’s backup sights and enjoy a long morning of shooting. Besides installing backup sights on all of my rifles, I store an extra set of batteries in each rifle’s Magpul MOE aftermarket pistol grip (pictured right), and keep extra batteries in my range bag.

Stay away from cheap Chinese made “Airsoft” quality parts and gear, such as Aim Sports, Promag, Leepers, UTG or NcStar. Save your money and buy quality gear. Your life may depend on it. As always, when you install any parts, be sure to use Loctite thread locker on the threads of the screws to keep your parts from loosening up when you shoot.

Manufactures of Quality Backup Sights Include:

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Potential Problems Mounting and Zeroing BUIS:

99.9% of backup sights are considered to be mil-spec height above the bore because they are intended to be mounted on mil-spec rifles. You will run into problems if you try to use Mil-Spec height rear backup sight with a non Mil-Spec height FSB.

Mil-Spec Front Sight Base = Non Issue:

Mil-Spec front sight bases for A-3 (removable carry handle / flat-top) AR-15 rifles are taller than older A-1 and A-2 (fixed carry handle) rifles.The way to tell whether or not you have a mil-spec FSB is that it is marked with an engraved or raised “F” on the left side of the front sight base’s forging (as seen in the photo to the right). There may be other markings to differentiate the company of origin, but a “F” on the left side of the front sight post is the only way to be sure. If you have an “F” then you can simply add your rear backup sight and/or optic and you will be ready to sight-in the rifle. You can skip this section.

Front and Rear Backup Sights = Non Issue:

If you have a rifle that has neither a front or rear sight, but does have a rail to mount both front and rear (as seen in the photo to the right), then you simply purchase a pair of quality front and rear backup sights and mount them on the rifle, making sure to use Loctite thread locker (If you don’t use Loctite then I promise that your sights will eventually work themselves loose.). The following potential issue does not pertain to you. You can skip this section.

Issues With Rear Backup Sight On Rifles With Non Mil-Spec FSB:

There are a few manufacturers whose “commercial grade” rifles are equipped with FSBs that are not quite exactly Mil-Spec height. Bushmaster still uses the older, traditional shorter A-2 front sight base on their A-3 (removable carry handle) rifles, as do DPMS and several other manufacturers.

Technically the all front sight bases are the same height, but the top part of the FSB where the post extends from sits higher than a standard “commercial” FSB, as seen in the photo to the right.

To make up for shorter FSB, Bushmaster, DPMS, and some other manufacturers use shorter removable carry handles. The difference between a Mil-Spec and commercial removable carry handle is very difficult to tell apart unless you have one of each to compare, or have a dial or digital caliper to measure. Remember, a non Mil-Spec carry handle will not work with a “F” marked Mil-Spec height FSB, and as stated earlier, a Mil-Spec carry handle will not work with a non “F” marked “commercial height” FSB.

You will not be able to co-witness your backup sights with a red-dot or holographic sight if you so choose. Remember that with the vast majority of backup sights that all elevation changes will be made with the front sight. You will not be able to zero the rifle because you won’t be able to raise the front sight post enough without the front sight post backing out of the FSB (as seen in the photo to the right). Completely unacceptable.

Pictured right is a comparison of the Mil-Spec front sight post (left) that sits on a taller front sight base compared to the .040″ taller post (right) that is required on the shorter “commercial height” sight base to make it work with backup sights. If you are going to mount a backup rear sight on a rifle with a fixed, “commercial height” FSB, then you will need to replace the original front sight post (pictured left in the photo to the right) with a slightly taller (.04″) front sight post (pictured on the right in the photo to the right) that can be ordered from Windham Weaponry and shipped for around $10.

Replacing the original front sight post will require a front sight tool AR-15 front sight tool that can be ordered from the same company. Simply use the tool to simultaneously hold in the front post retaining pin while unscrewing the sight post out of the sight base. It’s very easy with the tool, and frustrating and time-consuming without it.

It is interesting to note that replacing the front sight post is more critical on Bushmaster carbines, but may not be necessary on full-length rifles with 20″ barrels.

Before you start ordering parts for replacing your removable carry handle or front sight post for your “commercial grade” rifle, verify that it is not marked with the raised “F”. Older Bushmasters were not. There is a chance that since Bushmaster is under new ownership that newer rifles may have the correct height Mil-Spec front sight tower and sight post that will work with Mil-Spec backup iron sights.

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What Distance to Zero Non-Magnified Optics & Backup Sights:

If you are going to use BUIS without any type of optic then you will need to decide what distance to zero your rifle.

Battle Sight Zero = Set It And Forget It:

Simply defined, “Battle Sight Zero” (sometimes referred to as “Battle Zero”, “BSZ”, or “BZ”) is a theoretical “set it and forget it” setting for your backup sights or non-magnified optics that will allow your to make combat effective hits out to a certain distance without applying any hold-over or hold-under from your point-of-aim. With your sights set to a “Battle Sight Zero” you will know that your rounds will hit no more than “X” inches above or below your point-of-aim from CQB distances out to “Y” yards or meters distance. You will want your sights set at a distance that will provide the flattest trajectory, and thus the least deviation in point-of-aim and point-of-impact at varying distances. Fortunately for those using .223 Remington / 5.56mm NATO rifles, there is a distance that you can zero your rifle and enjoy a very flat trajectory out past 200 yards away.

What Zero Distance Provides The Flattest Trajectory?:

It is important to zero the elevation of the sights with a single setting that will take the most advantage of the flat shooting trajectory of the 5.56mm/.223 caliber rifle round. The following diagram compares the trajectories of bullets when zeroed at 25, 50, and 100 yards. While you can expect minor variations in trajectory depending on if you are using a carbine with a 16″ barrel or a full-size battle rifle with a 20″ barrel, as we as with different brands, loads, and weights of ammunition, the data supplied is accurate enough to compare trajectories for use with backup sights and non-magnified optics.

100 Yard Trajectory:

The first chart shows the trajectory of a 5.56mm NATO round when fired from an AR15 style rifle. The dash line represents the shooters point of aim. The bullet departs the barrel approximately 2.5″ below the point of aim. It’s flight path reaches the shooters aiming point at 100 yards (point-of-aim (POA) = point-of-impact (POI). The bullet then drops back down to 2.5″ below POA at 200 yards, and plummets drastically after that. This isn’t a terrible zero if you never plan on shooting past 200 yards as the bullet will impact somewhere within the distance of the barrel to the top of the rifle’s sight base. Not terrible, but you can do better.

25 / 300 Meter Trajectory:

The next graph represents the traditional military method of zeroing the rifle at 25 / 300 meters (meaning that you zero the rifle at 25 meters and can expect a second POA = POI at 300 meters). This was traditionally done with adjustable rear sights, such as those found on rifle’s equipped with carry handles, set on the 300 meter setting, but the target set 25 meters away. This is a TERRIBLE zero setting. At 100 meters (109 yards) the your rounds will impact over 4″ above your POA. At 175 meters the bullet impact will reach its peak apogy at roughly 6″ above your POA. The USMC has finally accepted that this isn’t the most efficient way to zero an AR15 / M16 style rifle. Do not use a 25 meter “set it and forget it” zero.

50 Yard Zero:

The next graph illustrates the ballistic trajectory of a 5.56mm NATO round with a 50 yard zero (not meters). This zero is commonly referred to as “Improved Battle Sight Zero”. As with the graphs above, you can see that the bullet leaves the rifle 2.5″ below the point-of-aim (POA). The bullets trajectory will pass through the shooters point-of-aim at 50 yards. At 100 yards it impact approximately 1.5″ high. It will reach its peak apogy of 1.8″ at approximately 140 yards. Around 220 yards the round will again pass through the shooters (POA). At 250 yards the round will impact approximately 2.5″ below POA. This data shows that on its flight from the rifle’s muzzle out to 250 yards, the bullet will hit somewhere within plus or minus the height of the rifles front sight base (+/- 2.5″). That’s pretty darn flat.

50 Yard, 25 / 300 Meters / 100 Yards Trajectory Comparison:

The final graph compares all three trajectories. The 100 yard zero isn’t terrible, but you can do better. The 25 / 300 meter zero is terrible. The 50 yard zero will provide the combat shooter with the flattest trajectory out to approximately 250 yards, and is the recommended distance to zero your backup sights and non-magnified optics as a “set it and forget it” Battle Sight Zero setting.

Conclusion:

The 50 yard zero provides the flattest trajectory for the AR-15 style rifle. It is easy to see why the 50 yard zero is the best choice, and how your point-of-impact will never be more than +/- 2.5 inches from your point-of-aim, from close up and personal “bad breath” distance, all the way out to a distance of approximately 250 yards. Competitive shooters and the USMC now realize this and employ the 50 yard zero technique with backup sights and non-magnified red-dot and holographic optics.

Important: This Battle Sight Zero is for AR-15 style rifles firing 5.56mm NATO ammunition. It is not necessarily an appropriate Battle Sight Zero for other 5.56mm rifles (because they may have sights of different height over bore than the AR-15), or rifles in other calibers. This is not the correct zero for Tavor, Steyr AUG, or AK rifles chambered in 5.56mm.