The Primary Arms GLx MD-21S Red Dot Sight (part number 810025, SKU PA-GLX-MD-21S) is a duty-grade 21 mm micro red dot sight with a 2 MOA red dot, 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 in the box.
Solar Failsafe is what separates the GLx from the value line. A photovoltaic panel on the top of the sight supplements the battery in daylight, so the cell is doing less work exactly when the dot needs to be brightest — and AutoLive shuts the dot down when the rifle has been still and brings it back the instant you pick the rifle up. Between them, the practical result is a sight you leave switched on permanently and rarely think about.
A 2 MOA dot is the precision-biased choice: it covers about two inches at 100 yards, so it does not obscure a small target the way a 6 MOA dot would.
How It Compares
This is the solar-backed GLx with a plain 2 MOA dot. For a ring-and-dot reticle in the same body, see the GLx MD-21S with the ACSS-CQB reticle. Without the solar panel and at a lower price, the SLx MD-21 2 MOA red dot sight keeps AutoLive. For a 25 mm tube sight with a rotary brightness knob and the Aimpoint Micro footprint, see the SLx MD-25 Gen II. If a red dot looks smeared to you, a prism scope may suit your eyes better — see the Primary Arms prism scope range. Browse all rifle dot sights and red dot sights, and read our red dot size chart and co-witness height guide.
Ideal For
- Civilian Service Rifle and practical rifle, where a 2 MOA dot keeps precision at 100 m
- Straight-pull AR-platform rifles in Great Britain wanting an always-ready sight
- Practical shotgun and 3-Gun, where speed and durability both matter
- Night fox and vermin control, using the night-vision-compatible brightness settings
Key Features
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Reticle: 2 MOA red dot, covering approximately 2 inches at 100 yards
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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 dot 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 |
2 MOA red dot |
| 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 / 810025 / 818500018278 |
What's Included
- Primary Arms GLx MD-21S 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 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 dot needs to be at its brightest.
What does AutoLive do?
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 on permanently 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.
What mounts come with the GLx MD-21S?
The Primary Arms GLx MD-21S is supplied with both a low mount and an absolute co-witness riser with spacer, so it can be set up either low or in line with iron sights on an AR-pattern flat-top receiver.
Is the GLx MD-21S night-vision compatible?
Yes. The brightness range on the GLx MD-21S includes dedicated night-vision-compatible settings, so the dot can be used behind a night-vision device without washing out the image.
How does the GLx MD-21S differ from the SLx MD-21?
The GLx MD-21S adds the Solar Failsafe panel and a higher overall specification, while the SLx MD-21 is the value model with AutoLive but no solar backup.
