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Magpul M3 300BLK Magazine Review

Magpul PMAG GEN M3 300BLK Magazine Review [Updated 2022]

Here is a review “Regular Guy Guns” did on a couple of Magpul PMAG GEN M3 300BLK magazines he picked up from The Mag Shack:

Sorry (but not sorry) about the political and social posts over the last few weeks. To be honest, I felt it necessary, especially since if we don’t retain our rights, it’s going to be kind of hard for me to provide review content on a regular basis. So, without further ado, I’m really proud to present my thoughts on the Magpul 300BLK PMAG – aka the PMAG® 30 AR 300 B GEN M3™.

First, a little history on the 300 Blackout cartridge. Developed by Advanced Armament Corp during the Kevin Brittingham days in cooperation with Remington, 300 BLK, as it is known, was designed to address some specific needs within the US military’s special operations community. Most notably, they wanted better terminal performance (terminal meaning how the bullet performs upon impact with the target), than 5.56mm, and also superior subsonic performance than 9mm rounds out of a submachine gun. In other words, a rifle cartridge would also obviate the need for certain special operations troops to carry an SMG rather than a rifle. Furthermore, the new cartridge was required to work out of a standard M4 (AR) lower, use standard 30-round AR-pattern magazines, and not require a special bolt.

.300 AAC Blackout vs 5.56 NATO vs 7.62x39

.300 AAC Blackout (x3) vs 5.56 NATO vs 7.62×39

For decades, ARs had shipped in “non-standard” configurations, where an upper would be chambered in something such as 7.62 x 39mm, which is the familiar standard cartridge for the AK platform. While working “AK” ARs exist, they are often plagued with feeding issues due to the extreme curvature of the magazine, and also the bolt carrier group is non-standard, limiting the ability of the owner to source replacement parts.

After much research, AAC and Remington decided to use the wildcat (wildcat is an “ammo nerd” term for a cartridge that is not standardized) 300 Whisper cartridge as the basis for the new caliber. After some minor tweaks and modifications for consistency amongst loads, 300 BLK was born in 2010. Able to be loaded into standard AR magazines, converting an existing AR into 300 BLK only necessitated a barrel change, satisfying the military requirement for easy upgradeability.

How? Simply put, 300BLK has the same outer case diameter as 5.56mm, with the cartridge itself not being “necked-down” as severely to accept a .30 caliber projectile. Ballistic requirements were satisfied as well, with the new load having similar, if not better, terminal effects than 7.62 x 39mm, and superior terminal effects at close range than 5.56mm.

And of course, subsonic variants suppress like a dream, since the round was developed in part by a suppressor company.

SIG MCX Rattler supressed

SIG MCX Rattler Canebrake suppressed in with .300BLK PMAG

So, if 300BLK is designed to work in standard 5.56mm AR magazines, why is a dedicated 300BLK magazine needed?

Why a 300 BLK PMAG?

While, for the most part, you average civilian shooter, including myself, has never encountered a problem with 300 BLK in standard 5.56mm magazines, that cannot be said for the military. While regular guys and girls like us maybe crank out 2-3000 rounds a year through our rifles, usually in semi-auto, the hardcore special operations community plows through ammunition at a far higher rate, with more full-auto use, of course. So, their likelihood of encountering a failure is much higher – with 300BLK in a standard magazine, they did.

According to Magpul, certain unnamed units requested specialized magazines for their 300BLK rifles. Thus, the PMAG® 30 AR 300 B GEN M3™ was born. Based on the GEN M3 PMAG, the new magazine boasts a specific internal geometry for the various 300 BLK projectile weights and profiles. While 5.56mm projectiles go from (on average) 55 to 77 grains and are usually of similar geometry, your 300 BLK projectiles range from 147 to 220 grains and boast a wide variety of geometries. Some of these can hang up in a standard PMAG, on occasion.

Also, on a more practical level, the 300 BLK PMAG looks and feels different. This is very important, even to us regular guys and girls. Why? Since the case geometries are similar, a 5.56mm AR will chamber a 300BLK projectile. Now, even though it is chambered, it does not mean the bullet will successfully be fired. Far from it. 5.56mm is roughly equivalent to .223, i.e. .22 caliber. 300 BLK is, of course, .30 caliber. The projectile will not fit. However, the trigger will slam the BCG (and firing pin) forward, and strike the primer. The end result is a very dangerous “ka-boom”, which at the least, will wreck your upper and damage your lower. The potential for injury, and even death, is high.

With this in mind, Magpul designed the new magazine to feel ergonomically different from a standard PMAG. For one, it’s smoother, and there’s a different rib pattern. Picking one up blind, alongside a normal PMAG, I could tell the difference.

In not so many words, reliability and safety were the driving forces with this magazine. It might not seem like a big deal, but the proper magazine is critical for your rifle’s functionality.

The real-world test

For a few years, I’ve owned a 300BLK AR. When I acquired my first suppressor, the Silencerco Specwar 762, it made sense to build a 7.62mm caliber rifle to really take advantage of the suppressor. And with 300BLK being touted as a “must-have” for a suppressor junkie, it was the logical step. But until now, I only used standard AR magazines. My “safety” was simple. I used red electrical tape as a visual indicator at the range. But, even with that, there was the possibility of an accident, especially if I bought multiple rifles with me that day.

The opportunity to evaluate this new magazine came about, at random, when The Mag Shack reached out to me to give this magazine a spin. Sorry it took so long, guys. The Mag Shack, out of Tallahassee, caters extensively to all types of shooters, from professional door kickers, to people like myself. It’s great to have cool gun businesses here in Florida.

Magpul PMAG 300BLK review

The PMAG 300BLK loaded in an AR

Anyways, after some kicking around in Instagram chat, they shipped me a sample to evaluate.

And of course, I called on my friend with the select-fire M16 lower, who helped me evaluate the Daniel Defense DD Magazine a while back. The ideal stress test for a rifle magazine is with a select-fire weapon.

So, after some planning and ammo-shopping, we proceeded all the way down to the famous Henry’s Range in Homestead. Henry’s is a no-frills outdoor range off the beaten path in the Everglades, which proved more than adequate for our testing purposes.

Of course, I started out with the basics, i.e. loading up the magazine. Typically, in my experience, loading 300 BLK into a 5.56mm magazine is relatively straightforward, but on occasion, you have to fiddle with the ammunition to get it to feed properly. This time around, I had actually forgotten my UpLula loader at home and had to load up the hard way, with my hands. Surprisingly, even without a tool, the 300 BLK ammo loaded extremely smoothly. Even with the last few rounds, which usually present a challenge with any magazine, loaded without effort. 30 rounds, check.

The first course of fire was a mag dump of supersonic 300BLK in my semi-auto AR. Loaded and made ready, and fired. Zero issues, over the course of maybe 30 seconds of fire. I deliberately dropped the magazine with force and reloaded. Another course of semi-auto fire. Zero problems. Drop test, check. It’s durable, like any other PMAG.

Testing a PMAG 300BLK

Testing the PMAG 300BLK

Now for the fun part, full-auto magazine dump with subsonic ammo. For the task, I loaded up the magazine with some Armscor 208 grain subsonic 300BLK ammunition. Made by Armscor, a Filipino company, in their US plant, the 300BLK offering uses a Hornady AMAX projectile and brass sourced from Norma of Sweden. I’ll wager the propellant is IMR. Diversity, in cartridge form.

Anyways, much like our experience with the 70s vintage M16 lower and the Daniel Defense DD Magazine, we expected some jostling to get the PMAG to load properly. Alas, this was not the case. After mating my 300BLK upper and Silencerco Specwar can to the classic machine gun lower, we were pleasantly surprised to see the 300 BLK PMAG loaded effortlessly into the tighter magwell of the full-auto weapon.

The results speak for themselves.

As you can see, we ran the magazine in full-auto a few times. Zero failures, zero problems. It’s solid from where I sit. Also worth noting is sending about $40 worth of ammo downrange each time we did a mag dump. Full auto has its applications for both civilian and professional use, but at the range, it’s money into (quiet) noise.

For those who are curious, our “mish-mash” firearm had a cyclic rate of around 792 rounds per minute. My friend did the math as follows:

subsonic – 30 rounds – 00:00:04,593 – 00:00:06,866
2273 milliseconds / 30 rounds = 75.76ms per round.
60 seconds = 60 * 1000 = 60,000ms
60,000ms / 75.76 = 791.974656810982049 rounds per minute

The time stamps are calculated from the second part of my short video, and he derived the RPMs from there. Totally much-appreciated nerdery.


Casual shooters of 300 BLK could go through life without this magazine. If one rarely uses their 300 BLK AR, then it could be seen as unnecessary. However, I beg to differ at all levels. Unlike the DD Mag, where you pay a hefty premium for +2 rounds and a super-grippy ergonomic construction, the 300BLK PMAG is only a few dollars more than a standard PMAG. If you can afford a 300 BLK build, and the ammo (which is still rather pricey…), then you can afford a dedicated PMAG or two. If anything, you’ll get major ammo nerd points.

Another huge thanks to my new friends over at The Mag Shack, of course – you can pick up the Magpul 300BLK PMAG here.