Channel: Brownells, Inc.

Category: Sports

Tags: gas portgas system lengthrifle-lengthoperating systemdwell timegas tubear-15brownells quick tipgas systemgas pressurecarbine-lengthcycling malfunctiondirect-impingement gas systempistol-lengthmid-lengthchoosing a gas systemgas blockdiagnosing cycling problem

Description: Brownells Gun Tech™ Caleb Savant dwells on a topic that's important when you're diagnosing a cycling malfunction on an AR-15 or choosing the gas system for a new AR-15 build: dwell time. "Dwell time" is the amount of time the combustion gases from the fired cartridge act on your AR-15's operating system. When you fire a round, the bullet is propelled down the bore by hot gas. After it passes the gas port, some of the gas escapes through the gas block into the gas tube, which takes that gas back to operate the bolt-carrier and cycle the action. If you web search "AR-15 dwell time," you'll get a lot of hits on a different answer, and Caleb explains why it's not entirely accurate. You'd need ultra-high-speed laboratory instruments to measure actual dwell time, so a simple indicator of dwell time is GAS SYSTEM LENGTH (the distance from the chamber to the gas block). Different gas system lengths provide different dwell times. Shortest to longest, the standard AR-15 gas system lengths are Pistol (about 4.5"), Carbine (~7"), Mid-Length (~9"), and Rifle (~12"). For example, if you have two 16" barrels, one with a Carbine-Length gas system and the other with a Mid-Length, which one has the longer dwell time? Write down your answer and see if it matches Caleb's answer! Also, the closer the gas port is to the chamber, the higher the pressure of the gas that hits the gas tube. The farther the bullet travels down the barrel, the less gas pressure behind it. So a Pistol-Length gas system generates substantially MORE pressure than the much longer Rifle-Length system. The Big Takeaway: DWELL TIME is the total amount of time the GAS pressure from a fired cartridge acts on an AR-15's operating system.