The Primary Arms SLx MD-21 Red Dot Sight with the ACSS-CQB reticle (part number 810028, SKU PA-SLX-MD-21-ACSS) is a 21 mm micro red dot sight with the red ACSS-CQB reticle, powered by a CR2032 cell with 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.
This pairs the speed-focused ACSS-CQB reticle with the value MD-21 body. The reticle puts a fine centre aiming point inside a large outer ring: the ring is what the eye finds first under pressure, dragging your attention to the middle of the sight, while the fine centre point is small enough to shoot accurately with once you are there. It is a meaningfully faster reticle than a bare dot at close range, and a number of shooters with mild astigmatism find a ring easier to centre than a single point of light.
AutoLive shuts the sight down when the rifle has been still and wakes it the moment you move, so it can simply be left on.
How It Compares
This is the value 21 mm sight with the ring-and-dot reticle. For a plain dot in the same body, see the SLx MD-21 2 MOA red dot sight. For a Solar Failsafe panel and a higher specification with this same reticle, step up to the GLx MD-21S with the ACSS-CQB reticle. For a 25 mm tube sight with a rotary brightness knob, see the SLx MD-25 Gen II. If a red dot appears badly smeared to you, the glass-etched reticles in the Primary Arms prism scope range are the more reliable 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
- Airsoft and training use, where fast target acquisition is the whole point
- 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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Electronics: AutoLive motion sensing, shutting the reticle down when idle and waking it on movement
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Power: One CR2032 cell
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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 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-SLX-MD-21-ACSS / 810028 / 818500018261 |
What's Included
- Primary Arms SLx MD-21 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.
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.
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 switched on.
Will the SLx MD-21 ACSS co-witness with iron sights?
Yes. The absolute co-witness riser supplied in the box sets the reticle in line with the iron sights on an AR-pattern flat-top receiver, and a low mount is also included.
Is this sight night-vision compatible?
Yes. The brightness range on the SLx MD-21 ACSS includes dedicated night-vision-compatible settings, so the reticle can be used behind a night-vision device without washing out the image.
