The Primary Arms SLx MD-21 Red Dot Sight (part number 810027, SKU PA-SLX-MD-21) is a 21 mm micro red dot sight with a 2 MOA red dot, 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 in the box.
This is the value model in the MD-21 line, and the compromise is a sensible one: you lose the GLx's Solar Failsafe panel but keep AutoLive, which is the feature that actually changes how you live with a red dot. The sight shuts itself down when the rifle has been still and is awake again before you have finished shouldering it, so there is no switch to remember and no flat battery at the moment you need it.
A 2 MOA dot covers roughly two inches at 100 yards, which keeps the sight honest at range rather than blotting out the target the way a larger dot would.
How It Compares
This is the value 21 mm sight with a plain dot. For a ring-and-dot reticle in the same body, see the SLx MD-21 with the ACSS-CQB reticle. For a Solar Failsafe panel and a higher specification, step up to the GLx MD-21S 2 MOA red dot sight. For a 25 mm tube sight with a rotary brightness knob and four included mount heights, see the SLx MD-25 Gen II. If a red dot looks smeared or starburst to you, the glass-etched reticles in the Primary Arms prism scope range will suit your eyes better. Browse all rifle dot sights and red dot sights, and read our guides to the best budget red dot sights and co-witness heights.
Ideal For
- Civilian Service Rifle and practical rifle on a sensible budget
- Straight-pull AR-platform rifles in Great Britain wanting an always-ready sight
- Practical shotgun and 3-Gun, where speed matters more than a premium specification
- Airsoft and training use, where a robust dot at moderate cost is the right call
Key Features
-
Reticle: 2 MOA red dot, covering approximately 2 inches at 100 yards
-
Electronics: AutoLive motion sensing, shutting the dot down when idle and waking it on movement
-
Power: One CR2032 cell
-
Illumination: Side-mounted push-button controls, with night-vision-compatible settings
-
Size: 21 mm micro body
-
Mounts: Low mount and absolute co-witness riser with spacer, both included
-
Housing: Waterproof and shockproof aluminium
-
Warranty: Primary Arms Lifetime Warranty
Technical Specifications
| Product Type |
Micro red dot sight |
| Magnification |
1x, unmagnified |
| Reticle |
2 MOA red dot |
| 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 / 810027 / 818500018254 |
What's Included
- Primary Arms SLx MD-21 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 AutoLive do on the Primary Arms SLx MD-21?
AutoLive is Primary Arms' motion-sensing system, which switches the dot 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 without draining the battery.
What size is the dot, and what does 2 MOA mean?
The dot is 2 MOA, which means it covers approximately 2 inches at 100 yards or about 5.8 centimetres at 100 metres, making it small enough for precise shots while still being quick to find.
Will the SLx MD-21 co-witness with iron sights?
Yes. The absolute co-witness riser supplied in the box sets the dot in line with the iron sights on an AR-pattern flat-top receiver, and a low mount is also included for a lower sight line.
How does the SLx MD-21 differ from the GLx MD-21S?
The SLx MD-21 is the value model with AutoLive motion sensing, while the GLx MD-21S adds a Solar Failsafe panel that supplements the battery in daylight and carries a higher overall specification.
What is the difference between the MD-21 and the MD-25?
The MD-21 is a 21 mm micro sight with side push-button brightness controls, while the MD-25 Gen II is a 25 mm tube-style sight with a rotary brightness knob and the Aimpoint Micro footprint.
Is the SLx MD-21 night-vision compatible?
Yes. The brightness range on the SLx MD-21 includes night-vision-compatible settings, so the dot can be used behind a night-vision device without washing out the image.
