Rear BUIS and RDS or HWS Use With Factory FSB:
Most AR15 style rifles built in the past ten to fifteen years do not have a built-in carry handle / rear sight assembly, but rather a “Picatinny rail” that allows the user to have a choice as to mount holographic sights, red-dot sights, magnified optics, simple rear sights, or even a removable carry handle / rear sight assembly. These are referred to as “flat top” rifles (pictured right). In the past, anyone wanting to mount optics on top of their AR-15 style rifles were forced to use some type of adapter to mount their optics on top of the fixed carry handle. This set the optics too high to take advantage of the 5.56x45mm NATO rounds fairly flat trajectory, and it set the optic too high for the shooter to get a good cheek weld on the stock. The flat-top design allows you to mount your optics so that the aiming reticle is the same height above bore as the tradition fixed “iron sights”.
Some flat top rifles are sold with a removable carry handle that looks exactly like the older fixed carry handles, except that they can be easily removed with two knobs. Some rifles are sold without the carry handle as manufactures realize that most end users will want to remove them to customize their rifles with optics and accessories.
- FSB: Front Sight Base. The fixed, non-removable triangle shaped tower that the front sight sits in. May or may not be “Mil-Spec” height.
- BUIS: Backup Iron Sights.
- RDS: Non-magnified Red Dot Sight.
- HWS: Non-magnified holographic Weapons Sight.
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What Distance To Zero Your BUIS and Non-Magnified Optics:
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 apology 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.
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.
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Co-Witness Iron Sights and Optics?
Co-witness means that with your backup sights match up with your primary optic. Your aiming reticle will appear to bisect the top of the front sight post. It is easy to co-witness backup sights if you first zero your primary optic. After the primary optic is zeroed, simply raise the backup sights into shooting position. After verifying that the weapon is unloaded, aim the rifle at a point approximately the same distance that the rifle is zeroed (50 yards). Using a front sight tool for the front sight, and the adjustment knob for the rear sight, you will want to adjust your backup sights so that the reticle of the already zeroed primary optic is bisected by the top of the front sight post of the backup sight. Keep your co-witnessed backup sights folded down until needed.
As discussed earlier, if you are going to install any type of electronic aiming device, it would be prudent to install some type of backup sights. You will also want the backup sights to be zeroed at 50 yards. With the optic and the backup sights properly installed and zeroed, it will look like the aiming dot of the optic is bisected by the top of the front sight post. The sights and optic will then be considered “co-witnessed”.
The easiest way to co-witness your optic and backup sights will be to carefully zero your optic first on the range so that your bullets are hitting exactly where you aim at 50 yards. You want to zero your optic first because it can be more finely adjusted than most backup sights. Next you will zero your backup sights. If your optic is correctly zeroed then you won’t have to fire a single shot to zero your backup sights. With the majority of backup sight you can not adjust the elevation of the rear sight. You must use a front sight post adjustment tool to raise or lower the front sight until it appears, when you are looking through the front and rear backup sight, that the optic’s reticle is horizontally bisected by the tip of the front sight post.
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How To Zero Your BUIS and Non-Magnified Optics:
If you use the front and rear backup sights exclusively, or use a non-magnified red-dot or holographic optic then you will want to zero your rifle so that the bullet impacts where you aim at 50 yards. This will give you the flattest trajectory possible with 5.56mm (.223 Remington) ammunition fired from a rifle with sights that sit 2.4″ to 2.5″ above the center of the bore. For further explanation, skip down to What Distance to Zero Non-Magnified Optics & Sights.
If you are going to use a non-magnified red-dot or holographic optic, first zero your optic and then zero your BUIS. Your optic has much finer adjustments than the BUIS. As stated above, zero your optic so that the bullet impacts where you aim at 50 yards. Zeroing your BUIS is as simple as looking through your deployed sights with your correctly zeroed optic turned on. Adjust the backup sights so that the optic’s reticle dot is bisected by the tip of the front sight post. You don’t have to fire a shot to zero the backup sights, although prudence would suggest that you verify your zero on the range with your optic turned off. Keep in mind that since the backup sights do not have as fine of adjustments as the optic that you might not be able to get the sights perfectly aligned with the optic. This is okay. Just get it close enough. They are backup sights. They won’t be perfect, but they will be close enough that you can continue the fight if your optic fails.
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Possible Issues Mounting Rear BUIS & Optic With A Fixed FSB:
If your new flat-top rifle ships new from the factory with a backup sight installed then the follow information should not be relevant.
If you own a flat-top rifle and decide to add a backup sight and / or red dot sight (RDS) or holographic weapons sight (HWS), then there are some pitfalls that you need to be aware of.
If you buy a flat-top with a fixed sight base and you decide that you want to purchase a removable carry handle to use as the rear sight then there are some factors to consider. There is a specific sight height that is considered “Mil-Spec”. Not all rifles are “Mil-Spec”, and you will find rifle manufactures that use sight towers that are slightly shorter (sometimes referred to a “commercial height”, the distance almost unnoticeable to the untrained eye). If you try to use a detachable carry handle with a Mil-Spec height rear sight on a rifle with a front sight that is “commercial height” then you find it impossible to zero your rifle.
Is Your Front Sight Base Mil-Spec Or Not?:
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 a mil-spec removable carry handle and skip this section.
Issues With Removable Carry Handles 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.
You will not be able to properly zero your sights if they are mismatched. Remember that your 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 a mil-spec removable carry handle. 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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- Savannah Arsenal’s AR-15 / M4 / M-16 Page
- Savannah Arsenal’s Tactical Rifle Optics
- Savannah Arsenal’s AR-15 — Sights and Optics Considerations
- Savannah Arsenal’s AR-15 — Mounting Backup “Iron” Sights (BUIS)
- Savannah Arsenal’s AR-15 — Mounting Removable Carry Handles
- Savannah Arsenal’s AR-15 — Mounting Optics To A Fixed Carry Handle
- Savannah Arsenal’s AR-15 — Methods For Zeroing A2 Type Sights
I’m confused. I have a F-type front sight. I have an A3 flat top upper with picatinny rail. I do not want a carry handle based sight. I want a Iron flip top rear sight or maybe a red dot optic rear sight. I don’t know how to match the rear sight to the front f-type.
Pretty much any AR15 backup sight will be the appropriate height for a F marked front sight post. Also, you want to buy a red dot that is able to be co-witnessed to the sights. IMHO you don’t want one with a mount that only allows you to see the iron sights in the lower 1/3 of the optic’s window.
AR-15 — Mounting Backup Iron Sights