Jumping on the piston-operated AR bandwagon

Many agencies that have received M16A1s via the DRMS 1033 program have wanted to upgrade these rifles to carbine configuration, and to get away from the 1/12 twist rate to better address their needs. Many have purchased upper receivers and collapsible...


Many agencies that have received M16A1s via the DRMS 1033 program have wanted to upgrade these rifles to carbine configuration, and to get away from the 1/12 twist rate to better address their needs. Many have purchased upper receivers and collapsible stocks from various companies to obtain this goal. Now SIG SAUER offers its version of the AR, the 516, as an upper receiver assembly upgrade kit for existing rifles.

In 2010, my department hosted the SIG SAUER Armorer class. During the class we got a heads up that SIG was going to debut a new rifle at the SHOT Show the following week. I was there and had a chance to examine the new 516 and 517 piston-operated AR15/AR10 based rifles. I was impressed with some of the design features, but it seemed everyone but me was jumping on the piston-operated AR bandwagon.

Fast forward to January 2011. I received information from SIG that they were offering the 516 upper receiver as a kit. (I knew that Ruger was going to do the same with its SR-556, but Ruger uses 1/9 twist rates and SIG uses 1/7, which appealed to me.) The idea of upper receiver kits has come up from time to time in both the law enforcement context, as I stated above, and the civilian shooting world. In California many of my shooting buddies have registered AR lower receivers that they wanted to upgrade in various ways. Some are into piston systems and have been following the development of various systems with the hopes of retrofitting their rifles.

The SIG 516 kit I received came in a full size SIG SAUER rifle case. The case contained the complete upper receiver, owner’s manual, replacement recoil spring and buffer. I noted that it has a forged 7075-T6 aluminum flat-top upper receiver with traditional forward assist, shell deflector and dust cover that mates directly with the top rail of the forearm. The forearm is an aluminum-free floated SIG SAUER brand with Picatinny rails on all four sides. It has a 1/7 twist, 16-inch chrome-lined barrel with SIG’s take on a modified M4 contour. The upper receiver has a nice, smooth, black finish. The quad rail forearm was attached solidly, and indexed to the upper receiver via a protrusion in the rail that fits inside a cutout notch in the top of the receiver in a kind of tongue and groove arrangement. I liked this feature as it keeps the rail in proper rotational alignment with the upper.

The finish on the rail was more of a gray than the upper and had a rougher texture. Two female non-anti-rotation sling sockets are machined into the left and right side of the rail at the front and rear. The gas block is dark black and shinier than the receiver, and unfortunately showed machining marks on both sides. The gas block had a bayonet lug machined into the bottom with a short section of Picatinny rail machined into the top for attaching backup iron sights, or anything else you want to attach.

A three-position gas regulator is housed in the gas block and protrudes out the front for easy access for adjustment and disassembly. The regulator is threaded into the gas block and held in position by a captured spring-loaded plunger. This plunger has to be pressed down before the regulator can be unscrewed for cleaning. Its square-shaped forward section protrudes forward with a hole in the middle. By grasping this square section and rotating the regulator, the operator can select between various gas settings. The hole is for inserting a tool or a 5.56mm round — just in case you cannot rotate it by hand. The standard three-position regulator can be adjusted to the following settings: the standard position allows enough gas into the system to operate the rifle under normal conditions, an adverse position allows more gas into the system for when the weapon is fouled, or other conditions such as temperature are interfering with reliable function of the rifle; and a final setting is for use with suppressors to reduce the amount of gas into the system. SIG offers a four-position regulator that adds a setting to cut off all of the gas into the system. This setting is for customers with specific needs, such as the use of rifle grenades. The unit I was sent came with the four-position regulator.

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