This pigtail gas tube was designed to solve timing problems for over-gassed 7.5" AR pistols. The pigtail design causes the tube to simulate a more standard cycle time and helps remedy timing problems without patchwork like a heavier buffer, etc. This allows the shooter/builder to use standard Mil-Spec carbine parts for better reliability. It fits all barrels using a pistol length gas system with an outer diameter at or below 1".This pigtail gas tube was designed to solve timing problems for over-gassed 7-1/2 inches AR pistols. The pigtail design causes the tube to simulate a more standard cycle time and helps remedy timing problems without patchwork like a heavier buffer, etc. This allows the shooter/builder to use standard Mil-Spec carbine parts for better reliability. Features: Fits all barrels using a pistol length gas system with an outer diameter at or below 1 inches Unfortunately our Pigtail gas tube will not fit correctly under the LPSN (Lo-Pro Slope Nose Quad Rail) The diameter of the outside of the pigtail gas tube loops range from 1.46 to 1.56 inches For best fit, the minimum inner diameter of a quad rail or hand guard should be 1.65 inches