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The SIG 516 Pistol

By Christopher R. Bartocci

In the ever popular world of the AR-15, there are endless variations for just about any purpose you could imagine. The AR15 has become America’s rifle and is used from everything from competition, target shooting, hunting and self defense. According to federal law, the barrel must be a minimum of 16 inches. Normal barrel lengths are 16, 18 and 20 inches. However, there has always been the segment of the market who like the short barrels. These are 7.5, 8.5, 10.5, 11.5 and 14.5 inches. Normally, these are restricted. First by Federal law, you must apply with an ATF Form 1, get local law enforcement approval and pay the $200 tax stamp or have a NFA trust and go through the same type of registration and tax. You also have state law to comply with. If you are in a state such as New York, they are illegal regardless of Federal approval. For those who do not want to go through the ATF paperwork, there is another option, the AR pistol. This enables the shooter to get their desired short barrel (under 16 in) legally but there is a catch. The weapon cannot have a stock. The average AR pistol will have a modified receiver extension that will not accept a buttstock and usually have a 7.5 or 10.5 inch barrel. Normal calibers are 5.56mm or 300 Blackout. They can also be found in 7.62x39mm and some companies even make pistols in 7.62x51mm (.308 Win).

The short barrels (7.5 to 10.5) have different operating dynamics than the longer barrels. Much of it has to do with the distance from the chamber to gas port and the gas port to muzzle. The short barrels, for example the 7.5 inch has an extremely short distance of both. The distance from the gas port to muzzle creates the dwell time in which the gas is introduced to the operating system (piston external or direct gas) to the time the system depressurizes when the bullet leaves the barrel and enters the atmosphere. With a rifle length system in direct gas, the longer gas system plus longer dwell time enables the cartridge case ample time to contract to be easily extracted from the chamber. With the shorter system, the pressures remain much higher and the cartridge case is still contains high residual pressure when the bolt begins the unlocking and extraction process. This causes the extractor to work much harder. Along with this is a higher cyclic rate. To compensate for the short dwell time of pistol barrels caused by the short gas port to muzzle distance, the gas port must be larger to allow more gas in to reliably cycle the action. The external piston having the piston chamber and operating rod located in the gas block is more effective and reliable than the direct gas that causes gas to be funneled from the gas block, back into the gas tube and into the bolt carrier where the expansion chamber is. To run the direct gas system properly more time is needed from the gas block to the muzzle to keep the system pressurized until unlocking of the bolt occurs. The other major issue with using direct gas system on such short barrels is the time pressure curve of the propellant itself. The direct gas system was designed for a 20 inch barrel. The propellant has pretty much fully burned by the time it reaches the gas port providing the peak pressure at the time the gas is introduced into the gas port. With the short 7.5 to 10.5 inch barrel the propellant has not fully burned, this results in two issues. First being a significantly lower port pressure to operate the mechanism as well as unburnt propellant being introduced to the upper receiver via the gas tube fouling the mechanism. This leads to the external piston advantage. First no fouling introduced to the mechanism and; second, the expansion chamber is right at the port making use of the pressure’s highest point. Both systems require much more gas to be able to operate. They just do it a little different.

SIG first offered the AR pistols with the traditional direct gas impingement, the SIG M400 pistol. This was offered in a 11.5 inch barrel starting in 2011. This was basically the SIG M400 PDW with a modified receiver extension that would not accept a stock, qualifying the rifle as now a pistol.

In 2013 SIG offered a new product to their AR pistol line up, the SIG Brace. This attached to the pistol and offered increased stability. There is a sleeve that your firing arm goes through and is secured by a large Velcro strip. What this does is stabilize the pistol with the shooters arm preventing the rear of the pistol from moving. This certainly increased accuracy over a pistol without it. In 2013, the ATF approved the SIG Brace and it not only was sold on SIG pistols but sold as an upgrade kit. Not long after the product was introduced a large controversy was opened up pertaining to the legality of the SIG Brace. People were putting the SIG Brace up against their shoulder and firing the weapon. By using it in this form, the SIG Brace becomes a stock and the classification of the weapon changes from that of a pistol to a short barreled rifle. If the SIG Brace is used as designed, as just that, a brace, the assembly is legal but by using it another way it is illegal. The SIG Brace is provided the letter of approval from the ATF dated November 26, 2012. This controversy is well beyond the scope of this article but was necessary to mention due to the test and evaluation pistol being equipped with one. It is safe to say when the brace is used properly; the accuracy of the pistol is significantly increased. Other options are just a round receiver extension and others have attachments to work as cheek welds but do not have anything that could be used for a stock.

The SIG 516 pistol (P515G2-7B-PSB) is for all intents and purposes the SIG 516 PDW with a stock modification. The telescopic stock is removed and replaced with the SB15 Stabilizing Brace (SIG Brace). Starting with the lower receiver, the receiver extension is standard Mil-Spec length but modified to only accept the SIG Brace. No stock can be attached to the receiver extension. The buffer is an H2 containing two Tungsten weights and one steel weight. The lower receiver is manufactured from 7075 T6 aircraft grade aluminum and has been improved with two Quick Detach sling sockets on either side of the lower receiver behind the rear takedown pin. The safety is ambidextrous and proprietary to SIG as well. The safety is much larger and easier to manipulate even with the thickest of gloves. The pistol grip is SIGs proprietary grip with a storage compartment in the bottom of it. It has gripping grooves on the front and rear as well as a grainy texture on the sides to make it anti-slip. The trigger mechanism is a standard semi-auto only Mil-Spec trigger group that broke at 7 3/4 pounds. The bolt catch has a new profile that replaces the lower portion with an elongated lever that is much easier to actuate. It is also easier to use with heavy gloves. Located on the left side of the lower receiver is a easy to use ambidextrous magazine release button. One interesting thing noted on the lower receiver is the front of the receiver has a more A1 type pivot pin support rather than the A2-type reinforced pivot pin area. The trigger guard is the standard winter trigger guard which can be disengaged for use with winter gloves. On the rear of the inside of the lower receiver is a spring loaded detent which prevents wiggling between the upper and lower receiver.

The upper receiver is also manufactured from a 7075 T6 aircraft grade aluminum forging. The upper is proprietary to SIG. The handguard has an interlocking tab with the front top of the upper receiver. The upper has a forward assist assembly, fired cartridge case deflector as well as a ejection port dust cover. The receiver has a Mil-Std-1913 rail that is continuous from the rear of the upper receiver, to the top of the handguard as well as the top of the gas block.

The barrel is a hammer forged 7.5 inch barrel. The barrel has 6 lands and grooves with a 1 turn in 7 inch twist. Pinned to the barrel is the gas block which contains the four position gas valve. The valve is adjustable by depressing a spring loaded detent on the face of the gas block. Under adverse conditions the valve may be rotated by inserting a cartridge tip through the valve and rotating it. If there is difficulty with the detent, the base of a cartridge case can be used to depress the detent. The operating rod assembly includes the piston, operating rod spring, operating rod, piston spring retainer and spacer.

The handguard is proprietary and has quad Mil-Std-1913. It is a two part hand guard. There are two quick detach sockets, one on the front of the handguard on both right and left sides.

At the same time this firearm arrived, so did a new magazine from Daniel Defense. There are several polymer magazines on the market. Some good and some, well let’s just say they are not ones to bet the farm on. Polymer magazines not made properly will have feed lips spread causing the top cartridge to pop out with the slightest tap due to the polymer being too soft or the feed lips break due to the polymer being too hard. The most outstanding feature of the Daniel Defense magazine is that it holds 32 rounds instead of the standard 30 rounds. When fully loaded, the magazine inserts into the magazine well without any difficulty at all. The yellow high visibility follower is of an anti-tilt design. The magazine is manufactured from a proprietary carbon fiber reinforced polymer and as of this writing is only offered in black. There are textured surfaces for enhanced grip. The magazine will accept a US GI stripper clip loader. There are no sharp edges and there is a hoop on the bottom if one wants to attach 550 cord or some other rope-type to assist in easy removal from the magazine pouch. All that is required to disassemble the magazine for cleaning is a 5.56mm cartridge tip or suitable tool. Daniel Defense advertises the magazine to function with 300 Blackout ammunition as well.

The SIG 516 Pistol was tested with a variety of magazines to determine compatibility. This included the previously mentioned Daniel Defense magazine plus the Magpul Gen 3 PMag, Lancer AWM, Aluminum GI, Hex Mag, Ultimag Surefire 60 round magazine. In total 300 rounds were test fired through the SIG 516 pistol. Test firing was 200 rounds of ZQI Ammunition 5.56x45mm NATO (62gr FMJ with penetrator). This ammunition is manufactured by MKE in Turkey. This appears to be Mil-Spec ammunition and is very reasonably priced in today’s market. The additional 100 rounds was provided by HSM out of Stevensville, Montana. HSM does not make components but assembles ammunition in house. HSM is well known for high quality ammunition and this test verified that. The HSM load was 5.56x45mm 62gr FMJ. The brass is brand new production Winchester 5.56mm primed cartridge cases.

Due to this being a pistol, the range was 25 yards. The pistol experienced no malfunctions at all during firing. Being fired as a pistol, there was little resistance to assist in recoil but the pistol worked perfectly. The pistol was fired with iron sights. As expected the muzzle blast was significant. The pistol was fired using the SIG Brace. This author had some difficulty shooting with the SIG brace. Off hand, all shot were within the center of a police standard silhouette target. Shot groups did tighten up with the SIG brace removed and resting the front of the pistol on a rest. Perhaps the best way found to shoot the pistol with the best accuracy was to use a tight sling and push outward on the pistol with your cheek on the tube to allow proper eye alignment. The best results were obtained with this method. The one disturbing thing found was the enormous muzzle flash. This would surely light up a dark room. The pistol could have benefited from a Vortex flash suppressor. However these are modification the end user would make, not the manufacturer.

Final thought on the 516 pistol is that it is a well built and reliable weapon system. Although not a precision weapon, the pistol does show merits as a sort range home defense weapon. With it being as short as it is, it is easily maneuverable in small spaces such as those found in a home defense environment. Using a proper laser sight and perhaps a flashlight would make it even more suitable. The front and rear sight should immediately be replaced with a dot sight. Holding the weapon like a pistol did show difficulty in using iron sights. When a DI Optical DCL23 was installed, the accuracy increased and the pistol was much easier to use. With practice, one can get very effective with this firearm.

This article first appeared in Small Arms Review V20N9 (November 2016)
and was posted online on September 23, 2016

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