The SIG 556 is more of a refined AR-18 design, from my understanding. While the AR-15 blows gas directly into the bolt carrier group, the SIG uses a gas piston. This keeps heat and carbon fouling isolated in the gas system instead of blowing it back in the reciever. Since the gas tube of the SIG is likely much larger than that of the AR-15, more comparible to that of the AK, actually, this isn't a problem and if you believe what you hear, should help increase the reliability of the SIG in adverse conditions while decreasing the amount of maintence required. I say it is an interesting concept, but still a poodle shooter. I'd love to support SIG for reaching into the civilian market with something as politically risky as a military based semi-automatic rifle, but right now I don't have 12 Benjamins laying around for a poodle shooter. If I did though, it would be the SIG, even with the atrocious handguards.