A Short Barrel, Full Auto, Gas Piston AR-15
Sig Sauer 516PDW
By: Steve Tracy
The alpha-numeric model designation for Sig Sauer’s 516PDW short barrel rifle (SBR) denotes that their 516 gas piston offering is suitable for entry-type law enforcement work. The PDW is the acronym for Personal Defense Weapon. It is available as a semi-automatic or with the selector switch that rotates all the way around to fully automatic mode.
A short barrel allows the gun to get through doors, around corners, and over obstacles without snagging or the need to lift the muzzle too much off a horizontal plane. The fully automatic action allows several rounds to be placed on target very quickly when a threat needs to be stopped immediately.
The AR-15 rifle system is common today as a patrol rifle and officers throughout the nation have spent a considerable amount of time training with this weapon system. A short-barreled model like the Exeter, N.H. manufacturer’s 516PDW keeps the manual of arms consistent and limits the learning curve when the new firearm is adopted for use.
Gas Piston Operation
Standard AR-15 rifles utilize the direct-gas impingement system of operation as incorporated by Eugene Stoner’s original M16. As the bullet travels down the barrel, gas behind it is bled off into a gas tube that runs above the barrel. This gas directly impinges upon the weapon’s bolt carrier to cycle the bolt. While the system has worked well for decades, the hot gas entering the chamber area burns off lubricating oil and causes excessive build-up of carbon filth. Anyone who’s cleaned an impingement AR-15 after a long day of training knows how dirty they can get.
Gas piston systems have become more common and are offered by several manufacturers. Sig Sauer presents their piston version as the 516 series of rifles, such as the 516PDW and the 516 Patrol (with standard 16 inch barrel). With a piston operating system, the gas bled from the barrel pushes on a rod or piston, which in turn pushes on the bolt carrier to cause it to cycle. The piston operation keeps heat and grime out of the gun’s action. Therefore, the lubricating oil placed on the bolt/carrier in the morning is still there in the evening after a hard day’s training. Cleaning at the end of the day is also a greatly reduced task.
The 516’s gas piston system includes a three-position gas regulator located on top of the barrel, ahead of the front sight area. A small button is depressed to let the regulator’s knob rotate to accommodate suppressed, standard, or adverse conditions. A 5.56 NATO round’s bullet tip can be used to easily rotate the knob instead of using your fingertip. A four position knob is also available, which adds another notch to cut off the gas system to make the weapon into a single shot.
The 516PDW is available in either semi-auto or fully automatic models. I was able to test fire both a 7.5-inch, semi-auto and a 10-inch, fully automatic model. Due to the modular design of the system, the uppers and bolts could also be exchanged to allow the firing of each barrel length in both actions.
The 516PDW’s barrel is chrome-lined, phosphate-coated and made from military-grade steel. Its twist ratio is 1 turn in 7 inches of barrel length to stabilize bullets heavier than 55 grains. The chamber sports M4-style dual-feed ramps for smooth, round feed. A removable flash hider/muzzle brake is threaded on the end. The quad-aluminum rail free floats the barrel to eliminate interference with barrel harmonics.
The upper and lower receivers are made from 7075-T6 aircraft-grade aluminum with a hard-coat, matte-black anodized finish. An M1913 Picatinny full-length flat-top upper rail allows the simple installation and repeatable accuracy of a scope or electronic red dot sight. Sig’s own flip-up iron sights (post front and dual-aperture rear) come standard with the weapon and they work well.
The weapon weighs 6.5 pounds without a magazine and the Magpul MOE collapsible stock fits the PDW to any officer’s body size. The MOE grip provides an excellent non-slip surface and is one of the most comfortable AR-15 accessories available. Push-button style sling swivel mounts are located on both sides at the rear of the frame and on the front of the quad rail’s sides. The mounts provide for the use of left- or right-handed single- or dual-point slings.
The 516PDW also sports an ambidextrous magazine release. Sig is working on an ambidextrous manual safety, but the current guns come with a left-side switch only. Each 516PDW is supplied with the always reliable, clear-window round-count Magpul Pmag that holds 30 rounds of ammunition. A vertical grip or handstop is a necessary accessory for any SBR. Caution must be taken to prevent the forward offhand from accidentally coming away from the forearm and slipping past the muzzle when firing, especially in fully automatic mode.
The 516PDW field strips the same as every other AR-15 variant, which is to say it is simple and straightforward with the push of the two captive pins to the right that attach the upper to the lower receiver. The bolt and charging handle pull out of the chamber to the rear to gain access to the gun’s inside.
At the Test Range
Sig’s rifle manager Jerrod McDevitt provided the 7.5-inch 516PDW and Sig Sauer Academy instructor and SWAT team entry team leader Chris Cavallaro brought along his own 10-inch 516PDW. A case of 5.56 NATO ammo was brought to the bench of the test range with steel silhouettes waiting at 100 yards. The 7.5-inch gun was topped with Sig’s own Mini Red Dot sight and Cav’s firearm mounted another brand’s red dot.
Both guns were found to be 100 percent reliable as the case of ammunition dwindled down. While minimal bullet velocity will be lost with the short barrel compared to a standard rifle-length barrel, inherent accuracy is not diminished. The shorter sight radius may make it slightly more difficult to make long-range hits with the iron sights, but an electronic red dot sight doesn’t realize that your barrel is shorter.
I found hitting the steel plates at 100 yards almost boring with both guns. They just couldn’t seem to miss. Engaging all three targets in various order with several hits each just produced an assortment of musical clanging sounds. The left-most target featured an additional round, swinging threat/hostage target to make for a more challenging shot. The 516PDW still made the hits from a standing, unsupported firing position.
The 516PDW is just as capable of long hallway shots inside a high school for an active shooter scenario as it is for close-up entry work. The short barrel does not prevent the weapon from making hits at distances usually considered only for longer bore rifles.
After the monotonous ringing of steel with the two Sigs, we moved over to cardboard silhouettes at 7 and 10 yards. Torso double taps followed up by head shots were intuitive with the excellent ergonomics of the Sig 516PDW with the Magpul grip and stock. The red dot sights aligned naturally, comfortably, and quickly with the shouldered weapon’s line of sight.
Cavallaro’s personal 516PDW was set up for his left-handed operation and he had swapped out the factory muzzle brake for one made by Battlecomp. There was a noticeable difference between the two and the disparity was dramatic when fired in fully automatic mode. We swapped out the uppers and bolts so that both the factory and the slightly customized uppers could be fired in full auto.
Muzzle climb with the factory brake could be countered by leaning hard into the weapon and pulling the forearm downward as rounds let loose. The Battlecomp created more pressure waves and sideways noise, but it really helped keep muzzle rise to a bare minimum. Team members off to your side might not be happy with the additional blast, but as the shooter, you don’t suffer the increase in felt detonation, only the greatly reduced muzzle rise.
The trigger pull on the factory 516PDW is set at 7.6 pounds. Obviously, it’s not a target rifle or sniper style trigger. It is a bit heavy, but for entry work, a hair trigger just won’t do. When it breaks, the trigger is clean and has no over travel. It works very well and was found to be capable of controlled bursts of one, two, or three rounds with a bit of practice. The fully automatic 516PDW is able to dump a 30-round magazine in under three seconds. That’s a lot of firepower when a threat is turning his muzzle in your direction.
Sig Sauer has once again delivered another excellent weapon for task-specific law enforcement missions. The commonality of the AR-15 design for both training and operation make the 516PDW an obvious choice for those desiring a weapon firing more than a handgun round in a short barrel rifle. The improvement of a cooler and cleaner gas-piston operating system makes the 516PDW an exceptional choice for law enforcement professionals seeking a short barrel rifle for duty use.
The 516PDW is a reliable, accurate weapon in a very compact package and it is effortless to shoot. It is intended for close-up entry work, but is quite capable of consistent hits out to 100 yards and beyond. As an all-around fighting tool for law enforcement, it’s a very sound choice.
Steve Tracy is a 22-year police veteran with 20 years of experience as a firearms instructor. He is also an instructor for tactical rifles, use of force, less-than-lethal force and scenario-based training. He can be reached at firstname.lastname@example.org.