I recently picked up a set of Magpul MBUS front and rear folding backup sights. They retail for just at $100 for the pair and as such offer a very economical alternative to the typical metal backup sights that cost twice as much if not more. There are certain limitations to these sights to get them to this price but given the intended role of giving the shooter a fallback set of iron sights if their primary optic goes down those limitations are largely acceptable.
The sights are installed by pulling the threaded bolt out of the sight and sliding the sight into place down the rail. This means that the charging handle must be removed before installation on the rear, and any kind of light or other object on the 12 O'clock rail must be removed before the front sight can be installed. Both sights came with a little blue Locktite already on the screw but in my constant on/off of the sights for the pictures below I found that it didn't really hold. I would apply more Locktite for a sure installation.
The MBUS are spring-loaded and deploy when the forward body of the sight is pressed down. This can be done by pressing either of the ears on the sides, visible below, or by pressing down on the top of the body itself. Once deployed they are held in place by spring tension alone. Some argue that this allows the sight to be knocked down and still spring back into place, but I think that it leaves open the possibility that something could get stuck in the sight and cause it to not fully deploy.
Where there are comparison pictures below they are compared to the Ubiquitous Troy Folding BUIS which retail for $250+/- for the set.
MBUS vs. Troy Top View
MBUS vs. Troy Bottom View
MBUS tension screw
Blue Locktite visible.
MBUS vs. Troy Clamping Mechanism
Note that the Troy has a separate piece that the screw tightens down to clamp the sight onto the rail. The MBUS, being made out of plastic, relies on flexing the sight body to achieve the clamping. This could result in over-tightening of the screw which could crack the plastic sight body.
MBUS vs. Troy Folded & Deployed
MBUS vs. Troy Various Angled Views
MBUS Rear Aperture
Note that the small aperture does not appear to be truly centered in the large, leading me to believe that these are not same-plane. I do not know if this is the way they are designed or if this is a function of material.
MUBS Front Sight Post
The hole you see in the front sight is for a roll pin (included, in the small bag seen in the first picture on this page) which can be driven in to keep the front sight post from rotating. Since I haven't zeroed the rifle yet, the roll pin is not installed.
MBUS Front Sight Post
Another hole for the roll pin.
MBUS vs. Troy Rear Sight - Folded
Note that the MBUS extends further to the rear than the Troy as mounted yet extends further forward as well. I would be inclined to mount the MBUS one notch further forward as I could see the potential of hitting the release when running the charging handle.
MBUS Co-witness through Aimpoint.
MBUS Mounted Behind Surefire X300
The 12 O'clock mounting of a pistol light is becoming popular. Here's how the MBUS mates up. I had to mount it a little further to the rear than the Troy.
MBUS Rear Folded
MBUS Rear Deployed, large aperture in place
The small aperture on the MBUS folds forward when not in use.
MBUS Rear Folded, under magnifier
Even with the small aperture folded forward the sight still clears the magnifier.
