Depends on what the barrel maker specs for a gas system, man. Fulton Armory has some fantastic 16" DPMS-based barrels that run standard rifle-gas systems. survivalshop, here, has one. Some barrel makers run carbine gas systems on 16" barrels - if the gas port diameter on those isn't dead-on, that thing would kick you like a pissed off mule. If you've got your barrel questions answered, did you already decide on a specific barrel? If so, you're stuck with a gas tube length that barrel is made for, and it's a moot point on gas tube length. If you've only settled on a length for the barrel, and don't have it yet, then you still have options on the gas system that you choose, based on what the different makers offer. <thumbsup> Gas block - lo-pro, pic-rail block - what do you want, for function? AR-15 gas blocks all work, and it depends on the barrel maker for the right size. Fulton Armory uses .936" diameter on their stainless Criterion barrels. Spill all your questions in this thread - they'll get answered. Guaranteed.