The Primary Arms GLx MD-21S Red Dot Sight with the ACSS-CQB reticle (part number 810026, SKU PA-GLX-MD-21S-ACSS) is a duty-grade 21 mm micro red dot sight with the red ACSS-CQB reticle, a Solar Failsafe panel backing up the CR2032 cell, and AutoLive motion sensing. Brightness is set by side-mounted push buttons and includes night-vision-compatible settings, the housing is waterproof and shockproof aluminium, and both a low mount and an absolute co-witness riser are supplied.
The ACSS-CQB reticle is the reason to choose this version over the plain dot. It places a fine centre aiming point inside a large outer ring, and the two work together: under pressure the eye locks onto the big ring almost involuntarily and is dragged to the middle, while the fine centre point is still small enough to shoot precisely with once you are there. It is a genuine improvement on a bare dot for close-range speed, and many shooters with astigmatism find a ring easier to centre than a single point of light.
Solar Failsafe supplements the battery in daylight and AutoLive shuts the sight down when the rifle is still, so this is an optic you leave switched on.
How It Compares
This is the solar-backed GLx with the ring-and-dot reticle. For a plain dot in the same body, see the GLx MD-21S with a 2 MOA dot. Without the solar panel and at a lower price, the SLx MD-21 with the ACSS-CQB reticle keeps the same reticle and AutoLive. For a 25 mm tube sight with a rotary knob, see the SLx MD-25 Gen II. If a dot still looks smeared to you, the etched reticles in the Primary Arms prism scope range are the better answer. Browse all rifle dot sights, and read our guides to astigmatism and red dot sights and the ACSS reticle system.
Ideal For
- Practical shotgun and 3-Gun, where the outer ring buys speed on close targets
- Civilian Service Rifle and practical rifle at close and medium range
- Straight-pull AR-platform rifles in Great Britain wanting an always-ready sight
- Shooters with mild astigmatism who centre a ring more easily than a bare dot
Key Features
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Reticle: ACSS-CQB in red — a fine centre aiming point inside a large outer ring for fast close-quarters pickup with retained precision
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Power: CR2032 cell with a Solar Failsafe panel that supplements the battery in daylight
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Electronics: AutoLive motion sensing, shutting the sight down when idle and waking it on movement
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Illumination: Side-mounted push-button controls, with night-vision-compatible settings
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Size: 21 mm micro body
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Mounts: Low mount and absolute co-witness riser with spacer, both included
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Housing: Waterproof and shockproof aluminium
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Warranty: Primary Arms Lifetime Warranty
Technical Specifications
| Product Type |
Micro red dot sight |
| Magnification |
1x, unmagnified |
| Reticle |
ACSS-CQB, red — centre aiming point with outer ring |
| Illumination |
Side push-button controls, with night-vision-compatible settings |
| Power |
CR2032 with Solar Failsafe panel, plus AutoLive motion sensing |
| Size |
21 mm micro |
| Mount |
Low mount and absolute co-witness riser, both included |
| Housing |
Waterproof, shockproof aluminium |
| Warranty |
Primary Arms Lifetime Warranty |
| SKU / MPN / UPC |
PA-GLX-MD-21S-ACSS / 810026 / 818500018285 |
What's Included
- Primary Arms GLx MD-21S ACSS red dot sight
- Low mount, co-witness riser and spacer
- CR2032 battery, tools and manual
Warranty
Covered by the Primary Arms Lifetime Warranty.
UK Delivery & Support
This red dot sight is UK-stocked and dispatched tracked from our UK warehouse — no import duties and no customs delays — backed by UK-based aftersales support and the Primary Arms Lifetime Warranty.
Frequently Asked Questions
What does the ACSS-CQB reticle actually do?
The ACSS-CQB reticle places a fine centre aiming point inside a large outer ring, so the ring draws the eye quickly to the middle of the sight under pressure while the fine centre point still allows precise shot placement.
Why choose the ACSS-CQB reticle over a plain dot?
The outer ring gives faster target acquisition at close range because a large shape is easier for the eye to find than a small point, while the fine centre keeps the precision that a large dot would lose.
What does Solar Failsafe do on the Primary Arms GLx MD-21S?
Solar Failsafe is a photovoltaic panel on the top of the sight that supplements the CR2032 battery in daylight, reducing the load on the cell at exactly the times the reticle needs to be at its brightest.
What does AutoLive do?
AutoLive is Primary Arms' motion-sensing system, which switches the reticle off when the rifle has been still for a period and brings it back instantly when the rifle is moved, so the sight can be left on permanently.
Is the GLx MD-21S ACSS night-vision compatible?
Yes. The brightness range includes dedicated night-vision-compatible settings, so the reticle can be used behind a night-vision device without washing out the image.
Does the ring help if I have astigmatism?
Many shooters with mild astigmatism find a ring shape easier to centre than a bare dot, though the ring is still a projected LED. If a red dot appears badly smeared to you, a prism scope with a glass-etched reticle is the more reliable answer.
