|The end product. Each square is 1 inch.|
Initially I was going to build an AR pistol in 5.56 mm, but the more I read up on the ballistics from the 5.56 and compared it with the .300 Blackout, the more I realized that .300 Blackout offered significant advantages out of a short barrel. There were also other considerations such as reliability, and muzzle flash and blast.
For one thing, a 5.56 is hard to run reliably in a barrel less than 11.5 inches. This is not to say it cannot be done, but that it takes a lot more tuning. Conversely, the .300 Blackout was designed around a pistol length gas system and will reliably run in barrels as short as 7.5 inches.
Another factor that played into my decision was the flash and concussion from a short barrel weapon. I'm not sure exactly when a 5.56 reaches full powder burn, but the cartridge was developed for use out of a 20-inch barrel, and I'm sure full powder burn is probably within a few inches of that. On the other hand, the .300 Blackout reaches full powder burn at about 9 inches of barrel. Thus, .300 Blackout should have less of a "fireball" out of a short barrel compared to 5.56. Secondary to this is that the .300 Blackout loses less of its potential velocity and energy when shot from a shorter barrel, whereas the performance of the 5.56 suffers a gross decline in its ballistic performance from a short barrel. In other words, with .300 Blackout, one could expect close to rifle velocity and energy even from a 9 inch barrel.
As you can see, this build employed a standard mil-spec, forged upper and lower. I mostly used a CMMG lower parts kit (including the trigger and the A-2 style pistol grip), with two exceptions. I was having some issues with a Strike Industries magazine catch in another rifle, so I swapped the SI magazine catch for the CMMG catch, with the latter working much better. The SI magazine catch accordingly went into this build, which works fine. I also had decided to replace my Strike Industries Strike Switch ambidextrous selector on another rifle with a standard ambidextrous safety selector, so I used the Strike Switch in this build and, between the smaller A-2 grip and purpose of this build, worked much better.
The barrel is a cold hammer forged 9-inch barrel from KAK Industries featuring a Melonite finish, a 1:7 twist, and 5R rifling. I decided on 9-inch simply because that was the point that there was full powder burn and, reading people's experiences in various articles and forums, it seemed 8 to 9 inches was really the sweet spot for a short-barrel .300 Blackout weapon. I ended up using an Odin low profile gas block (also Melonite finished) and a Melonite finished stainless steel gas tube made by Spike's Tactical. My decisions on the latter two products was not "snobbishness," but I noticed when shopping around that there was generally only a small difference in price between the Odin gas block, and Spike's Tactical gas tube and similar products from "generic" manufacturers, so it seemed a no-brainer to go with the product from a reputable company.
The pistol buffer tube was a "no-name" from a local store. I used a standard carbine buffer and spring. I wanted an end-plate that could serve to connect a single-point sling. Again, shopping around, I discovered that the price for a castle nut and such a plate was as much or more as the Primary Weapons Systems Ratchet Castle Nut and Endplate set, which featured a quick detach sling point on the end plate. The other advantage I saw was that PWS ratchet castle nut would make it relatively easy to change out the tube if I later decided to convert this to an short barrel rifle (SBR) or switch to one of the pistol brace systems that use proprietary buffer tubes.
I do not have a present intention to use a sound suppressor with this AR pistol, so I decided to look at various muzzle devices. Initially, I was looking at various muzzle brakes or compensators, but realized from the reviews that they would simply aggravate flash and concussion. I finally decided on a linear compensator from Kaw Valley Precisions (KVP), which was very nicely machined and Melonite finished, and at a much more reasonable price than similar products from other manufacturers. It is available for both 5.56 and .30 caliber builds. From what limited shooting I've done, it seems to work as advertised.
I knew from the beginning that I would be using an M-Lok free float hand guard of some type. After shopping around, I found myself going back to Aeroprecision because they had a high quality product at a lower price point than anyone else: the Aeroprecision Atlas S-One handguard.
|Aeroprecision Atlas S-One Handguard (9-inch)|
|Comparison between the Atlas handguard (top) and a standard Aeroprecision free float handguard (bottom).|
My only disappointment with the Atlas S-One was that the fit with the upper was not quite as good as I had hoped. As you can see in the two photographs below, their is a bit of a gap between the upper rail on the receiver and the upper rail on the handguard, and a slight difference in height, even though the upper receiver is also an Aeroprecision upper.
|Atlas S-One Interface|
|Standard free-float handguard interface.|
To counter the front-heaviness inherent in such a weapon, as well as to help steady the weapon, I attached a single point sling of the bungee style to the end-plate. With that wrapped around my neck and shoulder, the weapon is suspended in a handy position. Raising the weapon and pushing out slightly against the resistance of the bungee cording does a good job of stabilizing the weapon. The sling is one that has been sitting in a box for several years, so I don't even know its brand.
I've only done some function testing so far, but it shot the ammunition (some Federal 120-grain loads) without any issues--a nice change from my other AR that took some tweaking with the buffer before I could get it to shoot reliably (you can read about that here). One thing that quickly became apparent was that standard AR iron sights are impossible to use with the weapon extended. Obviously, I could use them if I shouldered the weapon, but that is not how I intended this weapon to be used. Accordingly, I will have to see if I can find some pistol style sights that I can use as backup sights. I also discovered that it is slow and uncomfortable to use a red-dot with an AR riser. However, I believe that a lower mount will work much better.
This was my first attempt to assemble the barrel and gas system for an AR weapon. It went surprisingly well, and I'm pleased with the outcome.
Update (10/17/2018): I've continued to work on this project, so my current iteration is shown below:
As you can see, I've added a Holosun 515CM red dot sight and an SBA3 arm brace by SB Tactical.