Quick Take / TL;DR
Out of the box, this red dot is packed with a good number of features, including multiple reticles and multiple bases to make absolute co-witness or 1/3 co-witness extremely easy to accomplish.
This is great for anyone who wants a non-magnified red dot. My use case is going to be for a pistol caliber carbine. If you need magnification, this isn't the optic for you without an additional magnifer add on.
It's a great value for the price, it's a great featureset and comes in at a very reasonable $180.
What Is the Primary Arms MD-21?
The Primary Arms MD-21 is a micro red dot that exists beside Primary Arms prism optics. The main difference between the MD-21 and the Prism optics is that the MD-21 lacks an etched reticle. That means your red dot is always battery-powered, so make sure you always have batteries on hand.
The MD-21 is similar in form and function to the Aimpoint T2 and the Holosun ARO-Core. They all have their pros and cons, but where the MD-21 excels is value; it's got some really impressive features for the pricepoint.
Key Specifications & Features
| Key Specifications | |
| Reticle | 2 MOA Dot (Red) |
| Brightness Settings | 12 Settings (Night Vision Compatible) |
| Click Value | 1/2 MOA |
| Weight | 5 oz. |
| Battery | CR2032 3V Lithium Coin |
| Material | Aluminum (Hardcoat Anodized, Black) |
| Mount Compatibility | Aimpoint Micro / Standard Microdot Footprint |
| Included Mounts | Low Mount + Absolute Co-witness Mount w/ Spacer (1.41" or 1.535") |
| Attachment | M1913 Picatinny |
| Warranty | Lifetime |
Key Features
- Auto-live Technology – Shuts off reticle after 2 minutes of no motion; instantly reactivates when picked up
- Red Dot Bright™ Reticle – Ultra-bright 2 MOA dot with extended runtime
- Night Vision Compatible – Full NV compatibility across brightness settings
- Fully Multi-coated Lens – Enhanced light transmission and clarity
- Top Mounted Push Button Controls – Easy brightness adjustment
- Shockproof & Waterproof – Rugged aluminum construction for any environment
- Multiple Mount Options – Includes both low and absolute co-witness mounts
Build Quality & Design
Construction of the MD-21 is well done both the front and back glass are quality. The push membranes have a quality tactile feel, they are responsive and holding the + changes through the reticle options.
The adjustment caps are flush, which is great to ensure you don't accidentally twist them and mess up your zero. The adjustment slots are big enough to be able to use the rim of a shell casing to make your adjustments for your zero.
Glass Quality & Window Clarity
Looking through, you'll notice there aren't many thick walls or distractions on the outside of the optic, which makes for a nice view that doesn't block your peripheral vision.
Oftentimes, the emitter can stick up in the viewport and block your view in the optic, but the MD-21s is just a small raised surface on the left side of the viewport.
The glass overall is clear with maybe the slightest blue tint, it's got a decent size and the reticle is very easy to see. With the reticle set to medium and pointed directly at daylight white LED panels, I could still make out the reticle well enough to see without adjusting the brightness.
Dot Quality & Reticle Performance
I have to preface this section with: I don't see dots as clear in any reticle, period, except for the prism optics. So take this with a grain of salt, the chevron reticle is just a dot for me, but nothing that impacts the overall use of the red dot sight.
I'm also a bit of a simpleton when it comes to most red dots. I like the simple chevron dot, but I am giving the chevron + horseshoe a chance, doing some shooting with it.
Brightness Controls & Power Management
As I mentioned earlier, the brightness control/power membranes have a very good tactile feel, longevity of the pad itself will only tell in due time. But the optic comes with a lifetime warranty and Primary Arms has solid customer service.
The brightness settings transition quickly and are very easy to change between. The battery this uses is a CR2032, and while there's no explicit battery life claim, it's probably best to keep some extras on hand.
Access to the battery is easy with the twist-on side cap that covers the CR2032 battery compartment.
Mounting, Footprint & Compatibility
Mounting the MD-21 is very easy given it comes with two different height 1913 mounting options. One is an absolute co-witness, and one is a 1/3 co-witness. Changing the mounts is as simple as unscrewing four screws and swapping the mounting base.
There are really no quirks when mounting all the hex keys you need, which are included in the box. If you are like me and have a handful of Primary Arms optics, you have a handful of their "T" hex tools lying around.
So if you are a must-have backup irons guy, the MD-21 has to be covered with a height option that will work.
Zeroing Process & Adjustments
The zeroing process is extremely easy, the adjustment turrets are tactile, and with the .5" MOA click adjustment, it's easy to get the optic dialed in quickly without burning through rounds.
So far, the zero has held after a couple of hundred rounds, and I don't expect it to change given the turret adjustment system's overall design.
Zeroing this optic went pretty smoothly, just had to tinker a little out of the box it was low and left (I KNOW I KNOW but it wasn't me and this is a rifle).
Real-World Use & Shooting Impressions
I like this optic and how the side walls of the optic aren't distracting and don't take away from your sight picture. I am running this on the co-witness count just so I can get my head up on this 300 Blackout rifle.
The only criticism I have of this optic so far is the lack of an auto adjust for the brightness, so if you set your brightness inside and go out on a sunny day, be prepared to bump your setting a little higher. That's a pretty minor and pretty much a non-issue.
Live fire the optic has held zero and shown no signs of issues, I think this will hold up for the long haul.
Who This Red Dot Is Best For
This is great for a go-to gun, it works great in engagements that are 150 yards or less and you don't have major issues seeing red dot sights. If this is an extremely affordable option that includes all the features you really need, such as auto-lock, shockproofing, and waterproofing.
Who Should Skip This Optic
If you need magnification but don't want to add a flip-up magnifier, skip this option. This is a great red-dot sight, but they aren't known for their magnification. If you like the ability to change reticles, this is a great option.
That doesn't mean it's not a good optic, because it's been a great optic thus far and I highly recommend it for most red dot sight applications.
Price, Value & Market Position
The red dot market has been all over the place in recent years, and you can still find some cheaper than the MD-21, but you probably won't find a red dot with the featureset of the MD-21.
With multi-reticle, autolive, and multiple mounting solutions, the $180 price tag is very reasonable. Best of all, it holds zero, is as durable as other red dot options on the market, but doesn't break the bank.
Final Verdict
If you are looking for a quality red dot that doesn't break the bank but is also reliable, has a solid featureset, and gives you the ability to have multiple height options the Primary Arms MD-21 is a top contender. We are also going to be looking at the GLx version, which has a solar panel and a few other features in the future.
This optic is similar to the Sig Romeo 5 series, which also recently underwent an update. Personally, I prefer the MD-21's design and the larger pressure pads for the brightness controls.
But as for the SLX MD-21, I recommend it; it's even made our best red dot sights list, which we recently updated for the new year.
