Bushmaster 5.56mm ACR Semi-Auto Rifle
Jeff Quinn at Gunblast - Bushmaster, Magpul, and Remington have collaborated "to produce a thoroughly modern and combat-ready twenty-first century rifle". Uses a piston and two-position gas regulator, instead of pissing where it eats. Multi-caliber. No tools required to swap the barrel, bolt head, and magazine. Not cheap, though. MSRP: $2,685, for the base model, according to Bushmaster's ACR page (ACR = Adaptive Combat Rifle).