1) M193 Standard military cartridge 55 Grain Lead core boat tailed copper jacket bullet.
Gas flow determines recoil- to much barrel after the gas tube can increase gas pressure and recoil and can cause excessive gun deterioration and ware. To short of a barrel after the gas tube can cause functioning problems such as insufficient cycling glitches causing jams. Military AR guns are set up for a faster cycle rate still allowing for recovery after recoil so you can aim and make your next shot. For civilian barrel purchases of different AR barrels gas function is usually thought about by the barrel manufactures to a great degree so what you buy will work. Some have experienced problems but in a pinch an adjustable gas block will let you tailor the gasses so you have the lightest recoil and the gun still functions but you sacrifice cycle rate.Though there are exceptions for the most part worrying about this gas system stuff is not necessary and the barrel you buy and the matching gas system is not a problem.
Richard W. Norman Author of American Handbook on Guns Ammo and Freedom