Chad Wallace

July 12, 2025

Optics, Review, Gear

Primary Arms GLx MD-21

Chad Wallace

July 12, 2025

Optics, Review, Gear

Primary Arms GLx MD-21

ACSS CQB Review

It seems like nowadays everyone is making some sort of tube style red dot. Primary Arms is no different, except they seem to always be upgrading or bringing out new styles. The Primary Arms GLx MD-21 is one of their new red dots and it has some features not usually seen on the current generation of red dots.

This GLx MD-21 is in Primary Arms’ mid tier optics line. What does that mean for you? You get clearer glass with less tint, a multi reticle system, a solar fail safe, and multiple mounting options. Since the GLx MD-21 is a projected reticle, you may still have some starring out effect if you have an astigmatism. I have one and the MD-21 gives me a pretty clear sight picture. Or at least better than some other red dots that I personally have.

The GLx MD-21 is a tube style red dot. This makes it ideal for carbines or AR style pistols. Having 21 mm lenses gives it a decent field of view, Everything needed to adjust the little red dot is also placed around the optic. On the right side is the battery compartment for the CR2032 battery along with the horizontal reticle adjustment. On top is the solar panel and the vertical reticle adjustment. The reticle adjustments are large, have really good clicks, and adjust in 0.5 MOA clicks. I like the finer adjustments on the MD-21 over the 1 MOA ones found on other red dots. On the left side of the optic are the multi-use, soft touch, buttons. They are quite large and labeled “+” and “-”. Then on the bottom is a standard T1/T2 footprint for mounting. Primary Arms includes two mounting options with the MD-21. They are 1.41" or 1.535" mounting heights. I used the 1.41” one and it worked great. If you want something different, there are a bunch of other options available for the T1/T2 footprints.

Other features of the MD-21 abound. Of course it has Primary Arms’ autolive feature that turns the reticle on when moved and off when idle. This is also the first Primary Arms solar failsafe red dot. The solar failsafe helps with battery life and also will let the red dot function with a dead battery. Yes, it works without a battery also, at least if there is enough ambient light. Also included in the GLx MD-21 is a new super bright LED. You can probably see the reticle from space if the brightness is turned all the way up. Speaking of brightness, it has 12 levels and is night vision compatible.

The MD-21 doesn’t just have a red dot reticle. It is available with Primary Arms’ ACSS CQB reticle, which is what they sent me for this review. The ACSS reticle is a horseshoe style with a center chevron and bullet drop marking below it. The bullet drop compensation is made for 5.56 and .308 out to 600 yards, but it also works with other calibers. I like having the bullet drop compensation, but in a 1x optic, it is really small and hard to make out. If you were to run a magnifier with it, it would work much better. With that said, the GLx MD-21 also has the ability to use only portions of the ACSS reticle. By holding down the + or - button, it will cycle through reticle options. You can have the full ACSS CQB reticle with the bullet drops, just the chevron and bullet drops, just the chevron (basically a dot style reticle), or the horseshoe and chevron (my favorite). Having the ability to choose which reticle you want is great. When adjusting the brightness on the MD-21, the reticle will flash when you reach its maximum or minimum brightness. I like this function when in the bright sunlight, that way I know when to stop pressing buttons.

I ended up mounting the MD-21 on the Global Monolith rifle. For some reason I tend to mount different optics on this rifle, probably because I like shooting the rifle and it makes testing out optics easier. Either way, I like the MD-21 on it. This little red dot is super bright, which works great in the bright sunlight. Even though the BDC part of the reticle tends to blur together for me, it is still useful if I want to shoot past 100 yards. The solar failsafe works so well that I actually did some shooting with the battery removed. The large buttons make it easy to adjust the brightness, or even the reticle. I really liked how well the MD-21 worked for my application. But I am also a fan of 1x optics.

Primary Arms did a great job with the GLx MD-21. I was hard pressed to find anything bad with the MD-21. If you like red dot style optics, it is definitely worth looking into. The solar backup and reticle options make this a great option in the red dot game. If you just want a standard “dot” version, they have that too. You can check out the GLx MD-21 over at PrimaryArms.com

Firearms Insider Reviews - Key Points

Claim to Fame:

Quality Solar Red dot with ACSS reticle

Target Market:

Anyone wanting a solar red dot

Features, Benefits, and Specifications of this product:

Solar Failsafe and Autolive technology extends battery life
Fully multi coated lens
Side mounted push button controls
Ultra bright ACSS-CQB reticle with 4 selectable modes
Night vision compatibility
Shockproof and waterproof aluminum construction
Black anodized finish
Includes Low mount and Absolute Cowitness mount with spacer (1.41" or 1.535" Height)
Standard T1/T2 microdot footprint
Accessory Attachment: M1913 Picatinny
Battery Type: CR2032 3V Lithium Coin
Brightness: 12 Settings
Click Value: 1/2 MOA
Finish: Hardcoat Anodized
Night Vision Compatible
Reticle: ACSS CQB
Warranty: Lifetime
Weight: 5 oz.

What other aesthetic options or finishes are available?

Dot only version

What others are saying?:

Evan: 4 out 5 stars at PrimaryArms.com

Nice Optic

Optic is nice but I am a little disappointed with the chevron. The chevron is too small to make out so it just looks like a dot. I'll have to view it under some magnification to see how I like it. The horseshoe is pretty sharp even with my astig. Brightness settings are awesome although I can't speak on the night vision settings because I don't have night vision to test them. Build quality seems solid and it has some weight to it. Overall I'm happy with this optic and can't wait to run it.

Link to other reviews:

704 Tactical

Price point:

MSRP = $219.99

Retail = Same

I need it now! Availability:

Primary Arms or your local retailer

Our Rating:

Pros:

Solar backup
Multi-reticle
Standard T1/T2 mount
Clear reticle
Lightweight
Comes with various height mounts
Autolive on/off

Cons:

Reticle is small and hard to see

Score: 8.50 Great

Favorite Link: Global Ordnance Monolith Review

Chad Wallace

April 2, 2023

Review

Primary Arms SLx MD-25 gen 2 red dot

Chad Wallace

April 2, 2023

Review

Primary Arms has updated their SLx MD-25 red dot optic to a Gen 2. The Gen 2 gives you all the great stuff as the first generation, but it now includes Primary Arms’ AutoLIve. With the added AutoLive function, this makes a great red dot for either a rifle or carbine. There are two versions available, one with just a red dot, or one with an ACSS reticle. I received the ACSS version, so everything here will be on the MD-25 Gen 2 with the ACSS CQB reticle. Most of the specifications, except the reticle, apply to the dot only version of the MD-25 Gen 2 also.

Let’s start this out with an AutoLive explanation. The AutoLive function, of the MD-25 Gen 2, automatically turns the red dot “On” when you pick up or move the optic. It also turns the optic into a sleep, or “Off” mode, after a few minutes of no movement. A big advantage to an auto-on reticle/dot is that it is ready when you need it to be. The AutoLive also extends the battery life, since your optic isn’t “on” all the time. Whatever setting the brightness dial is set to, is the brightness that it turns on at. No readjusting necessary. I absolutely love the AutoLive function. Speaking of brightness, the MD-25 Gen 2 has 11 adjustments on a dial. From night vision compatible all the way up to super bright. The 2032 battery is also housed in the brightness dial. Primary Arms specs the battery life at 12000 hours on the medium setting, so 5 or 6 on the dial. Battery life might seem a little low to some people, but there is more information in the ACSS reticle. For comparison, the MD-25 Gen 2 dot only version has a 50000 hour battery life.

This particular MD-25 Gen 2 has the ACSS CQB reticle. If you don’t know anything about the Primary Arms ACSS reticles, they are usually a horseshoe line around some sort of chevron (think triangle). The horseshoe on the Gen 2 measures 65 MOA. The chevron triangle sits inside the horseshoe and is used as your main aiming spot. For close up work, just cover the target with the chevron. For more precise work, use the top point of the chevron. The ACSS CQB then has three dots below the chevron. These dot’s are for ranging, as is the bottom of the chevron. The reticle can work with a few different common caliber options as seen in the photo. I like the ACSS CQB reticle, but it is pretty small, so it can be difficult to make out the aiming points for different ranges. The MD-25 Gen 2 is still a red dot style optic. So this means that the image is projected onto the viewing surface. It is not an etched reticle like a scope. This really only affects you if you have an astigmatism, which I do. So for me, the chevron “starbursts” out and looks more like a dot. This is just something to take into consideration. If you have decent vision, the reticle will be very clear. For my use, the MD-25 Gen 2 is not a long range optic, so this doesn’t bother me at all. When shooting real close targets, an advantage to the ACSS CQB reticle is that you can figure out how far above a target you need to aim, to compensate for mechanical offset. Those dots below the chevron, as well as the 65 MOA horseshoe, also help for aiming at those close targets. You don’t have this option with a standard dot only reticle, which is why I feel the ACSS CQB is a better reticle option for an AR rifle.

The Gen 2 MD-25 is not one of those super tiny red dot optics. I’d say it is medium size. Not so small that it’s hard to see through, and not so large that it’s a tank sitting on your picatinny rail. It only measures 3” long, so almost half the length of those expensive “red dots”. The MD-25 uses 25 mm lenses, but the field of view is really good. Primary Arms says it rivals 30 mm red dots, and I believe it after looking through the MD-25. Even though the MD-25 isn’t a small red dot, it still only tips the scales at 6.5 ounces. It also comes with different mounting height options. So no matter what you put it on, you can get the sight right up to your eye level. If for some reason you don’t like the supplied mount, the MD-25 can be mounted on standard T1 And T2 micro mounts.

I’ve been really impressed with the MD-25 Gen 2. It is built super strong. I’m not sure I could break it if I tried. Even if I was to, it’s covered by Primary Arms’ lifetime warranty. The little capped turrets even have raised nubs to keep them protected. These protection nubs will also keep stuff from snagging on the turrets, or at least help. The little turret caps also are aggressively textured with an adjustment built into them for making sight adjustments. The mount is also super strong. Once clamped down, the MD-25 isn’t going anywhere. I wouldn’t hesitate to put this on a rifle that gets beat around some.

If you are looking for a rugged red dot, that’s not a dot, look into the Primary Arms MD-25 Gen 2 ACSS. This thing is a great size for almost any rifle. It has a ranging type ACSS reticle that even works with a .22 lr. The AutoLive gives you plenty of battery life, over a years worth, and it makes the optic “always ready”. Go check out the SLx MD-25 Gen 2 over at Primary Arms, It’s a good red dot style sight with a unique reticle.

Firearms Insider Reviews - 8 Key Points

Claim to Fame:

Larger window red dot

Target Market:

Those wanting an auto on/off red dot with a ranging reticle

FNBs (Features & Benefits of this product):

AutoLive functionality
Wide field of view
ACSS CQB reticle
Up to 12,000 hour battery life at medium setting
Uses one CR2032 battery (included)
Next generation ruggedized mounting system
Includes risers and spacers for mounting to 0.96”, 1.41”, 1.54”, or 1.64” central height
Compatible with standard micro mounts
Fully multi-coated anti-reflective lenses
12 Illumination settings, night vision compatible
3" long
Weight - 6.5 Oz.
Fog resistant
0.5 MOA adjustments
Lifetime Warranty

What other aesthetic options or finishes are available?

Dot Only version

What others are saying?:

5/5 stars on Primaryarms.com

This is my 5th or 6th scope purchase from PA. I've never been disappointed yet, so when this one was announced I quickly put it on pre-order to replace a Vortex SPARC that I've never been happy with. I bought this to go on a home defense bullpup shotgun. It is fantastic for this purpose. The big outer circle, with the small aiming dot in the middle, is perfect for fast sight acquisition, and the shake-awake feature gives you one less thing to think about in a crisis situation. The included height adjustment plates let you tailor the height of the sight to your own weapon and preferences. Just another area where PA beats the competition in quality, value, and price. With eleven brightness settings on this reticle, you see it outside in the brightest sun, all the way down to night vision compatibility. I usually prefer green illumination over red, but this sight is extremely clear and my old eyes have no problems with it. - Highly recommended.

Link to other reviews:

Lynx Defense

Price point:

MSRP = $199.99

Retail = Same, but Primary Arms runs sales quite often

I need it now! Availability:

Primary Arms

Our Rating:

Pros:

Clear
Bright, well lit, reticle
Autolive
Mounting options
Field of view due to larger physical size
65 MOA circle of ACSS reticle

Cons:

Hard to see bullet drop compensation marks
Battery life compared to Red Dot version

Score: 8.00 Great

Favorite Link: Walker Defense

Community Reviewer

June 28, 2020

Review

Swampfox Wolverine slide and Justice red dot combo

Community Reviewer

June 28, 2020

Review

Written by: Rusty Odom

This review is on the new Swampfox optic Justice 27mm red dot pistol sight and the Steel City Wolverine combo. Let me start by saying I am an average Joe shooter with a huge interest in firearms. I am always looking for the latest greatest item or something to improve on an older firearm to make me a better shooter. For this review I chose a Glock Gen 3 model 22 in .40 S&W to convert to 9mm. The firearm was purchased some years ago for around $295 and it was a local police trade in. I tossed it in the gun safe, knowing one day I’d do something like this to it.

The Swampfox Justice red dot and Steel City Wolverine slide combo. The slide is in a distressed green pattern and is available for the Glock 19 and 17 Gen 1-3 only. The slide comes with the channel liner already installed and with suppressor height fiber optic sights. They are in the combat configuration (rear sight in front of the optic), the slide is smooth with no machine marks on the outside, except in the area of the optic cutout. The threads on the mounting service were nicely tapped and debured. When mounting the optic it is a nice snug fit. It also comes with 3 sets of screws and Allen keys to mount the optic. The slide has great machining in the front for a good grasp to reload with wet or sweaty hands. It’s texture is just the right combination of aggressiveness, not to be uncomfortable when sharply grasping the gun. It has a lightning cut above the barrel, I assume to reduce weight. I have a concern of dirt or debris getting into it and causing issues in a real world situation.

The optic is clear and crisp, it has an 27mm window and is made out of 7075 aluminum. There is a Liberty version that has a smaller window of 22mm. These are duty rated and have a new and improved hood over their last model. It has a top load battery that takes the CR1632 battery. Not Chad’s favorite. It weighs 1oz without the battery, its height is 1.04”, length 1.78” and width 1.24”. It has the normal parallax free, fog proof and ruby coated lens.

It claims to have a greater battery life and new and improved shake a wake motion sensing illumination. The slide and optic are both for the RMR mounting option. Assembly of the slide is straightforward, just like your normal Glock slide would go together. A Brownells Glock completion kit was used for final assembly.

The initial sight in was done by co-witnessing the dot and the front sight post. Once the impacts were on target I did a ⅓ co-witness, raising the dot above the front sight post to declutter the sight picture. The slide had a few failures to feed, but after some lubrication and some dry fire, it ran flawlessly. The optic is easy to pick up, with some practice, for someone that is new to the red dot game. So far I have fired 350 rounds of 115 grain 9mm with no problems after the initial FTE. I was able to keep all the rounds in the 10 ring at 10 yards. Even with astigmatism the dot was clear with some adjustment of the intensity level.

In my honest opinion I do believe that this will make a great setup for someone that wants to put an optic on their Glock and doesn’t want to have their slide cut, or wants the benefits of having one frame with two different calibers. After my initial testing and what I know of some of the other optic I own from Swampfox, this appears to be a quality red dot. The slide is nice and clean, the battle worn or distressed coloring gives it some character.

Firearms Insider Reviews - 8 Key Points

Claim to Fame:

A quality optic red dot with a standard RMR footprint, with a cost half the price of others. An improved hood to protect the glass, as well as shake awake technology. It claims to be rated for law enforcement and duty use.

The slide is well textured, nice smooth and rounded corners. Assembly was simple, machining was good.

Target Market:

People who want to get into the red dot game and not have to have their original slide machines. Also those who have a .40 or .357 Sig that want to convert it to 9mm for cost reasons.