ASH Prometheus 6.05mm M4 Standard 363mm Tight Bore Barrel.  Fits the standard M4, SR16, SIG 551, AK102 and other AEGs.  This is different than the high end Prometheus barrel, but still offers you a quality tight bore barrel for less money.  6.05mm inner barrel diameter lets you use a variety of ammunition without any problem of jams.  A great airsoft barrel for the money!