March 2010

Piston-Packing 5.56s: Kel-Tec, Sig Sauer, and Ruger Redux

We liked Rugerís SR-556FB and the Sig Sauer 556 Classic SWAT, even with their price tags north of $2000. Kel-Tecís AR lookalike proved to be a pleasant surprise, but had one ďglaringĒ problem.

Thereís no doubt about it; the AR-15 has gone mainstream. Over the last several years, the growth in demand for black rifles has lured many manufacturers (including the big guys) to introduce their own versions of the venerable Eugene Stoner direct-gas-impingement design. Now that the market has matured, we are now seeing engineering departments introduce versions that are claimed to improve upon the original AR-15 through the use of gas-piston systems.

In January, we compared a new piston gun, the Ruger SR-556, to two direct-gas-impingement units, rating the Ruger an A- in the process. This time, we put the Ruger SR-556FB, $1995, apples-to-apples against two more gas-piston models, the Kel-Tec SU-16 CA, $890 as tested; and a Sig Sauer 556 Classic SWAT No. SIG556302, $2399.

In the original AR-15, gases are vented back through a gas tube to work the bolt and bolt-carrier group. The entire assembly works as its own piston. Since all of the gas is vented back to the bolt, fouling and heat build up in the mechanism. A piston system uses exhaust gas to work an exterior piston and operating rod assembly, greatly reducing the amount of spent gas that reaches the gunís action. In our tests all three of our guns ran with less fouling than we see with our regular AR-15s.

The SIG556 and the Ruger SR556 use adjustable gas pistons, allowing the user to change the piston setting to optimize its operation for different ammo and operating conditions. The Kel-Tec uses a fixed gas piston in its design.

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