Monday, 4 March 2013

BUIS: Back Up Iron Sights

Photo from CF Combat Camera
I have noticed a disturbing  trend; soldiers deploying to hostile areas with only a primary sight on their weapon. Those little rubber nubs, that get worn off within a week, on the C79 optical sight are not enough. The Canadian Forces issues a plastic fixed sight with a flip apertures which is more than adequate.  There is much higher quality back up iron sights on the market as well. In some cases, the Quarter Master may be out of the issue back up sights. However, most of these soldiers are dripping with personal purchased equipment but not a decent set of BUIS. I will assume it has never been impressed upon them the importance of a redundant sighting system.

Canadian Forces issued BUIS from Colt Canada
Fixed, dual apertures, adjustable and light weight.
A more than adequate BUIS;
so why don't more soldiers put them on their C7 or C8.
Photo by CM

CF issue BUIS mounted forward of optical sight.
 However, it is mounted backwards which reinforces:
"Do soldiers understand the importance of a BUIS?"
Photo from CF Combat Camera
When I first joined the military, we only had irons sights except for those designated few who mounted AN/PVS-2 Starlight scopes or snipers. So, I learn how to shoot with irons quite well as we had funding for ranges back then. Fast forward to the modern army, every soldier has an optical sight and/or red dot sight. I truly appreciate, like only a veteran can, the increased speed of engagement and the night vision compatibility of a red dot sight. I regard highly the longer range attributes and surveillance capabilities of an optical sight or magnifier.  However, this does not preclude the need for iron sights.

Has a Garmin Foretrex 401 ($175-200 CAD) but no BUIS.
Photo from CF Combat Camera

BUIS is an acronym for Back Up Iron Sights; an absolute requirement for a fighting rifle or carbine. This blog is written by armed professionals for armed professionals, whether military, LEO or private contractor. As a recreational shooter or competition shooter, your requirements for Back Up Iron Sights will be different, perhaps not existent.  We will rummage into why you need a BUIS, types or styles, criteria for selection, and zeroing your BUIS.

 BUIS is  a requirement in a operational environment.

Why do I need Back Up Iron Sights or BUIS?

Honestly, optical sights and red dot sights (RDS) designed and manufactured for combat duty are a ninety five percent solution. They will rarely fail, when treated correctly and maintained, and will hold a zero, which should be verified at every opportunity.
  • Sights can and will be shot or knocked off in a blast during contacts
  • Normal up 'n down and wear 'n tear of combat operations will damage sights
  • Batteries do run out of power
  • Sights can get fogged up, dust covered or caked in mud

Many of these issues can be mitigate with proper maintenance and a solid SLAPS or pre-mission check. There is that Murphy factor and chance favors the prepared. The rate of failure is proportional to need at that moment. The ability to provide accurate fire immediately is constant.

Incoming Fire and Optics: not a good combination.

Two is one, one is none. Having a night vision compatible sight and an IR Laser Aiming device is the accepted required redundancy for night operations. No one questions that. Why would it be any different for daylight operations? A properly zeroed RDS or optical sight and a set of zeroed BUIS are the required redundancy for day light operations.

This soldier has redundant adverse lighting conditions
set up but no daylight redundancy...
Photo from CF Combat Camera

BUIS come in front and rear configurations, just like normal sights. If you are running a rifle or carbine that has a fixed front sight as part of the gas block, then you will only have to acquire a rear BUIS. If your rifle is totally bare of iron sights and your primary sight is an optic, whether optical or red dot sight, then you will require a set of BUIS including a front and rear. My recommendation is to acquire both from the same manufacturer as they have been designed to function as a together.

Has a Camelbak BFM ($200-250 CAD) but no BUIS
Photo from CF Combat Camera

Types of Back Up Iron Sights (BUIS)

Back up Iron Sights range from the simple to the extravagant. But, BUIS really break down into two specific groups: fixed or folding. Each has advantages and disadvantages. Selection on style is highly dependent on your primary sight system.



  • Less working parts thus lesser chance of failure
  • Can be put into action immediately


  • Height of sight limits placement
  • Primary sight can be ‘busy’ with a constant fixed sight behind it.



  • Low profile
  • Height of folded sight compliments most primary sights
  • Primary sight is not ‘busy’ with BUIS


  • Sight must be ‘flipped up’ to be put into action
  • Slightly more complicated design due to folding mechanism

I prefer the folding style BUIS. As they are a redundancy and rarely used, I like that they are low profile and out of my regular line of sight (LoS) keeping my primary sight clear. I know some switched on operators that prefer fixed sights, mostly for the sake of immediate use. It really comes down to your primary sight and room on your rail. You decide for yourself.

The MBUS Gen 2 from MAGPUL is an excellent example of a light, robust rear BUIS.
Perfect for a carbine with a fixed front sight as part of the gas block.
Photo by MAGPUL

Criteria for selecting Back Up Iron Sights (BUIS)

The ability to co-witness is a requirement of back up sight; otherwise you will have to take the precious seconds removing your primary sight before you can bring your irons into action. Not really an option in a street fight or room to room action. Co-witness is the iron sights lining up through the optic and being able to utilize back up iron sights without removing equipment or canting the rifle.  There are two co-witness options: absolute co-witness and lower third co-witness. In absolute co-witness, front sight post and optic reticle are in the center of the optic field of vision (FOV). In the event of a primary optic malfunction, raise a folding BUIS and get to work. This system works well for CQB as the head is kept up and it complements the advanced technique of ‘shooting the tube’ used in extreme cases. The lower third co-witness is setting up the optic so the reticle and front post sight only occupy the lower third of the optic field of vision (FoV). In the event of a primary optic malfunction, you raise the folding BUIS and lower your cheek weld in order to engage targets. This system works well with high mounted optics. It has the advantage of keeping the optic FoV open. Co-witnessing is primarily for red dot sights and does not work with optical sights.

 Absolute Co-Witness Vs Lower 1/3 Co-Witness
 by Vuurwapen Blog

Since the ability to co-witness is impossible with optical sights, many solid firearms companies have come out with offset BUIS or offset fixed sights. These sights are on a 45° off-set and are rail mounted which is secured to any MIL-STD-1913 rail system. Usually right handed shooter will offset them to the right side. Left handed shooters will do the reverse.  Getting them into action is simple, cant the weapon to visualized the sight, focus them on the target and go to work. There are fixed and folding versions of offset sights as well.

45 Offset Sights 
Knight's Armament Company

 A good set of Back Up Iron Sights should mimic the capabilities and attributes of regular irons sights but in a smaller package. A small aperture or peep sight and a large aperture or close quarter sight are needed. Should your primary optic fail, how long will you operation or contact last? At what ranges will you be engaging the threat? These factors cannot be forecasted. So, the sight must be set up for both long range and short range.

A rifle sight is useless unless it can be zeroed to the ranges  required and shooter. So, a good set of BUIS will be capably of adjustment on four axes, left, right, up and down. Front sights adjust up and down.  Rear sights adjust left and right for zeroing; some will adjust for range as well. Thus, allowing you to zero them, prior to going on operations, and being able to deliver accurate fire. Additionally they must be robust enough to hold that zero until required.

If you are considering a folding BUIS, the mechanism for activation must be able to be operated quickly, easily and lock into place. Remember to practice wearing gloves as they are part of your personal protective equipment (PPE). I am not too worried about how fast they flip down as I am never in a rush to fold them, much like I am never in a hurry to speed holster my sidearm after I have used it.

The mounting system must be secure enough to keep the sights in place during the abuse of combat. The construction of the sight must be robust enough to survive the rigors of an operational environment. Finally, the modern spectrum of operations requires much ancillary equipment to be mounted on our primary weapons. It is a game of ounces. Ounces equal pounds and pounds equal pain.  So, a lighter weight BUIS is better.

SureFire Helmet Light $175-200 CAD,
HSGI Warlord V2 $275-300 CAD,
M203 Grip- $275-300 CAD,
Photo from CF Combat Camera

Zeroing your BUIS

Ensure your Back Up Iron sights are installed correctly and secured. For operational use, a little Loc-tite Blue on the any threaded attachment system is a little added piece of mind. Also, witness marks are useful to be able to visually ensure the security of any ancillary equipment, especially your BUIS, with a glance. A witness mark is a mark which indicates the place where two parts line up - or where they should. Usually, once ensuring proper placement and securing your sight, you simply draw a short line or dot with a Sharpie or paint marker.

Example of Witness Marks on a rear BUIS.
Photo by CM

I recommend a 25m Point of Aim (PoA) Point of Impact (PoI) zero for Back Up Iron Sights. This is called a 25/300 meter zero. This will keep you on a man sized target all the way out to 300 meter with minimal hold offs. There are many different schools of thought on zeroes. I am recommending this one for a few reasons:

  1. A 25 meter range is usually used to verify zero after hitting the ground on a deployment. So, the facilities will be available.
  2. It is a metric zero. Unlike the 25m/200 yard zero. And, therefore, better for range estimation and range finding.
  3. This zero is excellent for close work and adequate for long range work. For CQB, you will need to remember your sight over bore radius but you will be bang on for compound and building ranges should your primary sights fail. For longer range, you will have much longer reactionary gap to set sights that you do not have during CQB.
  4. Finally, these are backup sights, not primary sights. A 25/300m zero on redundant sights is more than adequate. Spend the limited time and rounds on perfecting your zero on your primary sight at operational ranges as you will be using this sight all the time.

Click onto Enlarge. 

Your zeroing requirements will be set by your operational needs and mission. And, should you have the opportunity to perfect your zero with your BUIS out to a 100m range. Take that opportunity. It is a far superior zero.

Soldier has the required day and night sight redundancies
 which includes a rear BUIS, in this case.
Photo from CF Combat Camera
A very realistic drill for range practices is a Optic Failure/BUIS Drill. Simply,  start with a loaded and readied carbine and your red dot sight switched off in any position. On audible or threat signal, transition to your BUIS and engage the target. Five rounds would be adequate. See how long that takes you to get your BUIS into action. Try it in four orthodox positions. Then, try it in un-orthodox positions. This drill is about live fire rehearsal and familiarity with equipment.

An C7A2 with EO Tech 522 and MAGPUL MBUS,
 set up for absloute co-witness. 

A good combat optic, whether red dot or optical, will rarely fail if properly zeroed and maintained. However, rarely is not always. Your team mates and fire team partner are relying on you to be able to provide accurate sighted fire at all times. Failure is not an option during a gunfight. Have good set of properly zeroed BUIS or Back Up Iron Sights.

Take care out there.


  1. Great many variation in the iron sight it help to shoot the good point on that pictures really it useful to practice the target.
    Iron Sight Firearms

    1. Good research and talking to other end users will help with the selection of a BUIS.

      The link you posted links to a Ohio business. I have no experience with this company so I cannot, and will not, comment quality products available.

  2. Good article. Well thought out. Some hunters could use this info. Even as a former grunt myself, I find when I'm hunting with my scoped 308 I want irons for the surprised deer when humping out from the hide. Granted, the odds of catastrophic failure won't happen in a non-combat situation until that 10 pointer steps out of the trees in front of me.