The bolt carrier is consistent with other designs used in piston-operated ARs. It has a “tombstone” like piston contact lug in place of a traditional gas key that does not protrude over the bolt cam pin. The rear of the bolt carrier is fluted and rounded, both aiding in reliable function. I was surprised to see that the bolt was a standard AR bolt complete with gas rings. I like the fact that the bolt is not a proprietary part, as bolts are known to break. A shelf on the front of the bolt carrier indexes with a standard AR charging handle to allow the bolt and carrier to be cycled like any other AR.
The push-rod gas system is a short stroke design set to standard AR carbine gas tube length. The idea behind a piston system is to isolate fouling and heat in the forward gas block area and out of the action. The idea is that a piston system is more reliable in severe conditions and operates cleaner. Excess gas is vented out of the front portion of the handguard to the rear of the gas block via vent holes in the piston rod.
The system has been called self-cleaning, but don’t let that confuse you. Fouling is still deposited in this area and it needs to be cleaned just like any other rifle. You can dissemble the gas system by unscrewing the regulator from the gas block and pulling out the one-piece corrosion resistant coated piston rod without field stripping the rest of the rifle. The return spring assembly comes out with the piston rod. The end of the gas piston impacts against the lug on the bolt carrier. Firing gasses are vented into the gas block, forcing the piston rearward against the lug on the bolt carrier, cycling the rifle.
The kit did not come with any sights. Fortunately I had a Laser Devices EOLAD on hand and installed it on the upper. I tossed on an old vertical for grip and grabbed an extra lower I had to put the rifle together. I took the rifle out to the range several times. The first day was cold and rainy. I inserted a magazine and pressed the bolt release. The first round didn’t chamber fully. I cleared the round and never had a single hiccup after that. Once sighted in, I produced some great groups from the bench and off-hand. I fired an assortment of ammo through this upper, including some old surplus from the 80’s all the way up to some fresh 75 gr. Hornady TAP. It fed and functioned with everything I threw at it. Unfortunately I didn’t have any steal cased Wolf to toss in it during this test.
I noticed while shooting the 516 upper that it is not at all as front-heavy as other piston AR systems I have shot over past couple of years. Recoil was smooth but was noticeably more pronounced during rapid fire. My groups from 25 yard hammers were further apart than what I usually produced. This may be inherent with some piston systems, and not something unique to the 516. I could see the excess gas being vented out the front, but this did not create any issues. Overall the upper was accurate, reliable, and a pleasure to shoot.
Cleaning the rifle afterword was a snap. The piston and regulator contained most of the fouling but cleaned up easily. The amount of fouling on the bolt and bolt carrier was much less than what I usually see in a direct gas system AR. None of this was a surprise, as this is one of the selling points for any piston system. At this point I did not see any indications of damage to the lower or stock extension caused by bolt tilt, which has proven to be problematic with some piston ARs. Only time will tell with the 516.
I did notice that the fit between the upper and lower was loose and the upper wobbled on some of the lower receivers that I installed the 516 upper on. This did not affect function, but it can cause issues with accuracy. I installed an Accu-Wedge on the lowers that did not match up tight and this solved the issue.
SIG is offering the 516 upper receiver kits in several barrel lengths to meet the needs of individuals and agencies. This kit is a quick and easy way to convert your existing rifle to take advantage of a carbine length short-stroke piston system.