Building a State of the Art .300 Blackout Pistol

When I started my trip into the world of the .300 Blackout I never guessed that I would end up discovering as much as I did about what would appear to be a well-established cartridge. The pistol/SBR class of AR weapons was an area that I had some experience in, but not with this caliber. Below is my selection of the best parts I could find and why I chose them.

U.S.A. –-(Ammoland.com)- The thing about my projects with the .300 Blackout is that they started out at 16”. The caliber, when it was introduced, was marketed with short barrels that were handy to use with suppressors. The hand-in-hand relationship between the suppressor and the .300 BLK is the same as peanut butter and jelly. The thing about PB&J is that many people readily eat the PB part with a spoon, much the same way as they own an AR SBR or pistol without a suppressor. The ‘jelly’ in our equation, the suppressor, isn’t always as accessible to the average consumer as we would like, and most people I know don’t eat jelly with a spoon unless they’re a backward savage.

I digress, but my point is the .300 BLK unsuppressed in a pistol/SBR is a handy and effective means of self-defense and a friend on the trail.

My original articles in this series focused on the supersonic ability of the cartridge out of a full-size rifle with a 16” barrel. I compared the cartridge in both a dedicated SIG SAUER MCX Virtus rifle and a custom-built AR-15. The results were interesting and I decided to go with parts for this build that were both attractive and functional.

Due to popular demand, the ‘bakelite’ dye job in the prior articles will again appear here. This isn’t the time or place to fully discuss the process of dying AR furniture, but I will sum it up by saying that it is easy and can be done on the stovetop with a bottle of Rite Dye and a big 5 gallon boiling pot. You’ll want FDE (not the white Sand color) Magpul furniture, Sunshine Orange Rite dye, and some tongs. Boil the water, add the color, and let the parts go for about ten-twelve minutes while stirring occasionally. You can leave them in longer if desired. The parts will come out ready to install, but will be hot!

For the upper and lower receivers, I decided to again use V7 Weapons. These are some of the lightest and most appealing receivers available today and I’ve been happy to use their products in this and other articles. Some other products making another appearance are ScalarWorks sights, PRI Gasbuster charging handle, a Geissele two-stage trigger, and Faxon lightweight bolt carrier. These parts have proven themselves to me over and over again and I am fully convinced of their utility, lightweight, and reliability.

The upper receiver of the pistol will be our first stop. Underneath the Midwest Industries handguard is another Faxon product. If you haven’t guessed by now, I am a huge Faxon fan and believe in the quality of their products. The 7.5” .300 Blackout Gunner Barrel has a 1:8 twist, 5/8-24 threads at the muzzle, M4 feed ramps, and is finished inside and out in QPQ Nitride. It readily installed and looked great.

Folks, let be tell you right now that this is one accurate barrel. Most of my testing with rifles occurs at ranges inside 200 yards, but occasionally I am able to push it further for long range builds and other such things. I shot this pistol out to 200 yards with both subsonic and supersonic ammo and it was nothing but a walk in the park. The average group size at 100 yards using SIG 125gr supersonic loads was 1.5” with a velocity of 1975fps. If you look back to my original 16” articles, you will see that the velocity for the same load was 2125fps, which is hardly a difference at all. Other loads tested delivered similar performance, with an interesting exception.

The handloads I used that featured a Hornady 125gr HPBT and 21gr of Hodgedon Lil’Gun powder. The short, pistol-length gas tube and light carrier made the action cycle violent and I had several blown primers and a badly stuck case. I immediately stopped using the load and pulled all the bullets. I didn’t imagine that the action would be as stressed as it was.

This brings us to an interesting design problem with direct gas guns and even some piston-type actions. Despite the ability of the Superlative Arms bleed-off gas block to do what it supposed to do and bleed off gas, the power generated by the handloads in such a short gas system was too much to handle. The same exact loads in the 16” barrel with a longer gas tube performed exceptionally well and were quite fun to shoot. Shooters familiar with the M1 Garand will note that the rifle is not safe to use with high-pressure loads or heavy bullets. I’d say that the best and most accurate M1 loads, like the Hornady Vintage Match 168gr offering, are the best and they exhibit mild pressure and mid-range velocity. Going for maximum isn’t always the best.

This concept proved true for my pistol build as well. Something that I noted, aside from a long gas system on my previous build, was that I didn’t have extra mass on my lightweight carrier. The LAW folding mechanism uses a bolt carrier extension to account for the added length of the folding assembly. This added mass coupled with a longer gas system is what led to the soft-shooting characteristics when using such powerful ammo.

In my previous articles, I lamented the fact that I considered many commercial .300 BLK loads to be somewhat anemic, but that was just the opposite case when I was using the caliber in an AR pistol. I found that a 125gr bullet at or just below 2,000fps was plenty of power out of a 7.5” barrel. My handloads for this article, which are detailed below, were far more mild than those of my full-size rifle.

I wanted to test more and more with this gun, but the punishing weather and my supplies could only let me do so much. As usual, take caution when reloading for any caliber, especially this one. The .300 BLK is of great interest in this size because I discovered that, if I didn’t care too much about velocity, I could load with a half-charge for subsonic and a full one for supersonic. Using my powder measure set at 9gr per throw of Lil’Gun, I could load 225gr subsonics and two throws for 18gr for 110 and 125gr supersonics. I took care to measure the charges on my scale several times and determined that it showed no perceivable difference in weight. In short, throwing 9gr twice showed no difference over throwing 18gr once. This was handy and it really speeds along my .300 BLK reloading sessions.

The last stop on this trip is the lower receiver. There isn’t too much different going on here than on any other AR pistol lower until you notice the Maxim Defense brace. This thing is- and I’m not reaching at all here- completely awesome and is, to me, a necessary addition to any AR pistol. Hell, I’d even say it should be a required addition. The thing just works so well that you’ll have an identity crisis as to why you even SBR’ed your pistol in the first place. There is no perceivable advantage to an SBR when you have this on your pistol in my mind. Call me crazy, and I just may be, but the extra $200 to the government when you could just have this is ludicrous.

The brace is essentially a pistol-legal version of the company’s PDW stock. It has the same features with the added feature of being able to be strapped to an arm. The brace features four points of collapse and a closed length of just over 5 1/3”. On the sides, there are two QD sling attachment points.

The spring and buffer assembly included with the brace I received is special and shortened as compared to a regular carbine unit. The buffer, despite being significantly shorter than standard, maintains an H2 weight. It is available in different weights from Maxim.

The comfort and function this brace delivers is second to none and I wouldn’t hesitate to recommend this stellar piece of gear to anyone looking for a way to make their AR pistol just that much more functional. The best part is that, should you decide to SBR your pistol, you can simply swap out the brace for a Maxim stock and maintain all the functionality while getting NFA points with your friends.

All in all, I love this thing. In total honesty, this may very well be the best AR that I’ve ever built. The complete package weighs less than four pounds loaded and fits in my pack easily. It handles like a regular rifle and delivers better accuracy than other comparably sized weapons could ever hope to. The gun shoots easy and carries even easier. I believe that this combination of parts delivers the most function for the size and dollar and each is worth a look to the discerning shooter.

Knife in the feature photography is an ESEE Camp-Lore PR4.

Special thanks to:

• Brownells
• Maxim Defense
• Faxon Firearms
• V7 Weapons
• Hornady
• ScalarWorks
• Midwest Industries
• Sig Sauer
• Black Hills Ammunition

About Josh Wayner:

Josh Wayner has been writing in the gun industry for five years. He is an active competition shooter with 14 medals from Camp Perry. In addition to firearms-related work, Josh enjoys working with animals and researching conservation projects in his home state of Michigan.