Last year we witnessed a huge but short lived craze, the AR15 pistol with arm brace. What started it all was the advent of the Sig SB15 pistol stabilizing brace. Developed for disabled shooters to be operated one handed with the intention of making the pistol less unwieldy. The brace also inadvertently made a pretty good buttstock, turning an AR pistol into a quasi-legal short barrel rifle (SBR) without the NFA paperwork! The BATF originally ok’d shouldering the brace as they considered the act as not being a redesign of the pistol itself. This opened the floodgates for everyone that ever wanted a SBR but didn’t want the hassle or expense of the NFA process.
All was fine and dandy for a few months. AR pistols, a super low volume niche, were suddenly selling like hotcakes, breaking all sales records. Then as suddenly as it started the AR pistol craze came to a screeching halt when the BATF, bombarded with letters from gun owners and manufacturers asking if this brace was really legal, realized they created a loophole for people to get around the NFA process. The BATF then issued a statement saying that they considered an AR pistol equipped with a brace fired from the shoulder to be illegal.
Sales of AR pistols died overnight. During the rush, we did manage to obtain one to mess around with and despite the reversal, were able to create a super nice blaster. In the process, we have figured out cool ways to shoot it without becoming felons. You might find this interesting as well. Check out our AR pistol build and maybe you still might want to consider getting one.
The TRGRiQ Test Team for the day consisted of Joyce Leung, TiQgirl and Joanne Lee. Joyce and TiQgirl have have taken many civilian training classes and Joanne is a USMC Vet so they all have a lot of cumulative shooting experience.
As for a base gun, we chose the Primary Weapons Systems MK107 (mod 1) pistol. We went with the PWS MK107 because we wanted to build a really short, 7.5″ barreled pistol that functioned reliably. The mod 2 was not available at this time.
We wanted to try a piston operated gun as a super short barrel can be problematic for a direct impingement gas system. The short length of the barrel from the chamber to gas port and from the gas port to the muzzle means the gas pressure pulse needed to cycle the action arrives at the bolt carrier very quickly and is gone even more quickly. When the chamber pressures are high, they work on the fired case, keeping it expanded into the sides of the chamber, wedging it in place and making it harder to extract. The pressure pulse duration is also very short due to the limited distance from the gas port to the muzzle so there is not much time to tap pressure to power the extraction cycle. You can see in the picture above how much more pressure is in the bore with a 7″ barrel vs a typical 16″ barrel.
To compensate for this, most pistol sized barrels have large gas ports to try to ensure enough gas gets down to the bolt to cycle the gun. The problem with this is that it causes direct impingement pistols have really fast and violent extraction/ejection due to the short and really high pressure pulse going to the bolt. They also suffer from early extraction timing issues. Due to the short barrel lengths, the pulse gets there while the chamber pressures are high and the case is tightly wedged in the chamber. The extractor and ejector must overcome the pressure to remove the case. This is hard on the bolt, bolt carrier, bolt cam pin and extractor. Super short pistols are thus prone to getting stuck cases or having the extractor tear off the rim of the case as well as wear out the buffer and extractor springs rather quickly. They also have the reputation for being finicky and ammo sensitive. This is why it is usually advisable to build a 10″ > barrel pistol!
The PWS MK107 has a long stroke gas piston which is fashioned in the same manner as the AK47. The piston is up in the handguard right under the front sight. The gas system has a 3 position adjustable regulator where 1 is full open for typical conditions, 2 is for normal operation with hot loads, 3 is for using a suppressor with increased back pressure and 4 is for disassembly. The long stroke piston has more mass to retard violent extraction and the adjustable gas system is tunable to minimize stress on the guns internals. The long stroke system is simple and robust and not sensitive to fouling either.