The FAB Defense RBS is a rear back-up sight with a dual aperture design, built low profile and durable enough to serve as your primary iron if needed. It mounts on any MIL-STD 1913 Picatinny rail, which makes it a natural fit for M4, AR-15, and M16 platforms as well as FAB Defense’s own KPOS chassis. It works with any front sight you already have, so there is no need to replace hardware that is already on the rifle.
What the FAB Defense RBS Is and Who It Suits
This is a folding rear sight for shooters who run an optic but refuse to be left without a backup. Red dots and magnified scopes are outstanding until a battery dies or a mount takes a hit, and iron sights remain the only aiming system that cannot lose power.
The FAB Defense RBS is built from polymer, which keeps weight down without giving up the durability a working rifle demands. It is a sensible choice for a duty carbine, a home defense rifle, or a range gun that gets handled hard.
It also suits the shooter running a KPOS chassis or any modern sporting rifle with a flat top rail, since fitment is standardized and installation is quick.
Key Features
- Dual aperture design with instantly selectable 3mm and 1.5mm openings
- Mounts to any MIL-STD 1913 Picatinny rail, no proprietary hardware required
- Tool free windage adjustment for quick corrections at the range
- Ergonomic angel wing shaped shroud that helps your eye center the image
- Low folded profile that stays smooth and will not snag on gear or brush
- Polymer construction from FAB Defense, built to serve as a primary iron if needed
Specifications and What They Do For You
The two apertures cover genuinely different jobs. The 3mm opening is the fast one, suited to longer eye relief mounting, low light conditions, and close range speed work where a big, forgiving ring beats a precise one.
The 1.5mm aperture is the precision setting. It works best with shorter eye relief mounting and tightens your group at distance by giving the eye a finer reference to center.
Windage adjusts easily and quickly without tools, which means you can zero at the range with nothing in your pocket. That is a real advantage on a backup sight that might be pressed into service unexpectedly.
The polymer build keeps the sight light, and the folded profile stays inconspicuous so it does not intrude on your optic’s field of view or catch on webbing.
Installation, Pairing, and Care
Mount it as far to the rear of the receiver rail as your optic allows, since a longer sight radius always improves precision. Snug the clamp firmly and confirm it has not shifted after your first string of fire.
When paired with the matching FAB Defense FBS front back-up sight, the two create a precise optical tunnel for more accurate aiming and faster target acquisition. Pairing the set is the ideal configuration, but any standard front sight will work.
Zero the FAB Defense RBS separately from your optic so you know exactly where the irons hit when you fold the glass out of the way. A backup you have never zeroed is not really a backup.
Care is minimal. Wipe grit out of the hinge, avoid harsh solvents on the polymer, and check the mounting clamp during routine maintenance. If you need a matching front sight, a rail section, or an optic to sit alongside it, our optics and sights collection has the rest.
Bottom Line
A backup sight only earns its rail space if it is light, tough, and instantly usable. This one checks all three boxes and adds a genuinely useful dual aperture that most folding sights leave out.
Bolt it on, zero it, and forget about it until the day you need it. Ships direct to your door, no FFL required.
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