Join Date: Aug 2005
My Ride: F30 328i M Sport
| Backup Iron Sights (BUIS) use with Optics
BUIS is any type of fixed or folding sight is used as a secondary sighting system, and used as an alternative to an optic being used as a primary sighting system. While the term "Iron Sights" is used, BUIS refers to all sights, regardless of material. Most sights now are made of aluminum, and some are made of a polymer. BUIS are a dual-plane sighting system, consisting of a rear sight aperture and a front sight post. The front post is lined up into the middle of the rear aperture, which is why it's known as a dual-plane sighting system.
The common misconception with BUIS is that they work in RELATION to optics, as opposed to working separately from optics. This is muddied by misinterpretation of different types of co-witnesses. A co-witness means that you can use your BUIS without removing the optic, as the BUIS are visible through the optic. There are two types of co-witness: Absolute and Lower 1/3. Absolute Co-witness means that the center of the reticle is directly in-line with and on the same vertical plane as the line of sight through the BUIS. A Lower 1/3 co-witness puts the line of sight of the BUIS through the lower 1/3 quadrant of the optic, below the line of sight of the reticle. The reticle sits above iron sights.
The reason why people get this mixed up and have misconceptions is because of how the absolute co-witness works in relation to the BUIS, and how people use fixed BUIS, or have folding BUIS flipped up all the time. The misconception is that since they reticle lines up in-line with the rear sight aperture and front sight, it's meant to used in conjunction with those sights. People believe that since everything generally lines up, you have to make sure that you line all three up before ripping off a shot. This essentially turns a single-plane sighting system (dot, chevron, etc) into a three-plane sighting system by inserting it in-between a dual-plane sight. The legitimate use of an absolute co-witness is to have it so that you can use the same head position and cheek weld with the optic as you would with the iron sights. It makes for a consistent head position if you spend a lot of time switching between optic and sights. Ideally, absolute co-witness should be used with a folding rear sight kept in the closed/down position so as to not obstruct the field of view any further. Additionally, sights kept up will slow down the shooter, as it tends to force people to think about the alignment of the front and rear sight, when they should only be paying attention to the reticle and nothing else. Many people are running fixed rear sights when running absolute co-witness, and this is not an ideal setup. This is also an issue, because not every sight and mount combo lines up exactly with every BUIS.
This concept of absolute co-witness then creates confusion with new shooters when they're using a Lower 1/3 co-witness, because they believe that somehow there needs to be some kind of alignment and relationship between the reticle and the sights. In reality, there should be no intentional correlation or relationship between the optic and the sights. If you are running a fixed rear sight, or you prefer to run folding BUIS in the "up" position (defeats the purpose of a folding sight, but whatever... ), you should be running a lower 1/3.
I personally do not like the Absolute Co-Witness. It's problematic and causes a lot of confusion. People always seem to have to run it with sights up, which absolutely defeats the purpose of having an optic in the first place. Why use an optic if you're going to use the sights anyways? If you use it correctly, it works fine. However, a lot of people mess it up.
Another reason why I like Lower 1/3 is because of same-plane vs offset plane sights. Not all iron sights are same-plane. Same-Plane means that the rear sight aperture is directly on the same optical plane (in line with) the front sight post. An Offset Plane is where the rear sight aperture is higher than the plane that the front sight sits on. Offset plane is done for corrected longer range trajectory, which is why the A2 rear drum sight is designed as an offset plane. The A2 sight was designed to allow the iron sights to be used out to 800m, and to be able to use without adjustment form 0-300m. If you are running an offset plane with your sights in place or up, using an absolute co-witness becomes problematic because if you look through your sights, you won't be able to use the red dot properly. If you attempt to use the reticle, it will be obstructed by the outline of the rear aperture. Aside from A2 style sights, several folding BUIS have been identified as not being same-plane.
So to summarize, here are the benefits of each:
- Allows for an identical cheek weld between BUIS and optic
- Allows use of fixed rear sights, so that they don't obstruct as much field of view
- Ideal for a fixed front sight post (FSP), as it reduces how much of the target will be obstructed within your field of view
- Allows for a more "heads up" position when shooting standing up, so it causes less fatigue and more situational awareness
- Allows for a slightly higher head position in the prone, which is especially important if you have gear on your front like a tactical vest or chest rig; or body armor.
- No direct correlation between optic and sights, so there is no confusion
And to finalize this portion, I would like to reiterate that regardless of what setup you use for co-witness, the reticle of the optic should have no relationship with the sights in terms of your use. When you use the optic, FORGET ABOUT THE IRON SIGHTS!! Pretend that they're not there. Do not make any attempt to use them while your optic is operational, because they shouldn't have anything to do with each other. PERIOD. FULL STOP.
You should not be using an optic to deliberately use it with iron sights. The co-witness is merely the position where there sights sit when your optic goes down and you need to look through the optical housing when using the iron sights.
BUIS Use with Magnified Optics
BUIS used with a magnified optic, like a fixed or variable magnification rifle scope or a combat gunsight like a Trijicon ACOG, Browe BCO or Elcan SpectreDR will not co-witness with those optics. This includes a no-magnification/1x scope like the Leupold Prismatic, nor flash-dot scopes like the S&B PMII Short Dot or Leupold VX-R. You will not be able to use the sights through the optic. If you elect to have have BUIS on your weapon while using a scope or gunsight, you should employ a mount with a quick-detach (QD) feature like those offered by LaRue Tactical, American Defense Mounts or GDI. This will allow you to throw one or two levers and quickly remove the scope in case it goes down, making your sights accessible. If you are using folding BUIS, they should be stored closed/down. Ideally, you should avoid using a fixed FSP when running a scope or gunsight if at all possible, as well as avoid using fixed BUIS or running the sights in the up position.
An alternative to this is to offset-mount the iron sights. Offset mounting the sights usually places the sights at a 45° angle to the top rail. To use the sights, you simply rotate the rifle 45° to the left and sight through the sights. Knights Armament, SureFire, Diamondhead and XS Sights (among others) all make offset iron sights. It should be noted that offset irons are not wrong-hand (southpaw/lefty) friendly, as they mount to the right of the optic and rail.
See here for more on offset irons:
The alternative to this is to omit iron sights and run with a stacked or offset mini red dot. You can mount mini red dot sights like the Trijicon RMR or Leupold DeltaPoint on top of an optic, on top of one of the rings, or mounted along the tube of the optic. There is also the option of a 45° offset mount off of the rail, similar to the offset BUIS mounting. If you run a magnified optic, this is a more preferred method to run unless you are actually going into combat with the optic. At that point, run all three if you can. The offset 45° or stacked red dot option is wrong-hand compatible as well.
Offset DeltaPoint on GGG mount:
Offset Aimpoint T-1 on a LaRue Tactical offset QD mount:
"Stacked" Optic in the form of a Trijicon TA31-ECOS with a DOCTER optic mini red dot on top.
Trijicon TA31-ECOS with Trijicon RMR red dot. Note the additional integrated emergency iron sight on the left side of the optic.
Leupold DAGR (Dual Aperture Gunsight Riflescope) System, also known as the MDNS ECOS-O system (Miniature Day/Night Sight Enhanced Combat Optical Sight - Optimized) , which uses the Mark6 3-18x44 M5C2 TReMor2 with stacked Aimpoint T-1.
Last edited by Reedo302; 01-13-2013 at 02:07 AM.