Dwelling on dwell time
When guys decide to build an AR-15 from scratch, they usually pick their barrel length based entirely on how the gun is going to look or what kind of ballistics they want to push. They figure a sixteen-inch barrel is the standard do-everything length, so they buy one and slap it on the upper receiver. But a huge percentage of those builders completely ignore the most critical mechanical dimension on that entire piece of steel: the gas system length. If you mismatch your gas system length to your barrel length, you completely wreck the dwell time of the rifle, and your gun will beat itself to death.
To understand dwell time, you have to look at the exact sequence of events that happens inside the barrel when you pull the trigger. When the primer ignites the powder, the brass casing instantly expands. It swells up like a balloon and tightly grips the steel walls of the chamber. This expansion completely seals the rear of the gun so the explosive gas has nowhere to go but forward. As the bullet travels down the bore, it passes the tiny drilled hole called the gas port. The moment the base of the bullet clears that hole, highly pressurized gas shoots up into the gas block and travels rapidly down the stainless steel tube back toward the bolt carrier group.
But the bullet is still traveling down the barrel. As long as that bullet is still inside the bore, the entire system acts like a plugged pipe. The pressure remains incredibly high. The instant the bullet exits the muzzle, the plug is removed, and all of that pressure immediately vents out into the open atmosphere. Dwell time is the exact fraction of a second that the bullet spends traveling between the gas port and the muzzle. It dictates exactly how much high-pressure gas is injected into the action of your rifle.
If you take a standard sixteen-inch barrel and drill the gas port way back at the carbine-length position, you have nearly nine inches of barrel sitting in front of the port. That gives the rifle a massive amount of dwell time. The system stays pressurized for way too long. It violently over-gasses the action. The bolt carrier gets slammed backward with ridiculous force while the empty brass casing is still physically expanded and clinging tightly to the chamber walls. Because the brass hasn't had time to shrink back down, the extractor rips violently at the rim of the casing. You end up with torn rims, brutal felt recoil, and broken extractor springs.
To fix this, the industry introduced the mid-length gas system. By physically moving the gas port a couple of inches further down the barrel toward the muzzle, you shrink that remaining distance. You drastically reduce the dwell time. The bullet exits the barrel sooner after passing the port, venting the pressure earlier. The bolt unlocks smoothly, the brass has a millisecond to shrink away from the chamber walls, and the entire extraction cycle feels like it is running on ball bearings. When I am speccing out a new build and need to balance the dwell time perfectly, I pull my barrels, gas blocks, and tubes straight from https://www.brownells.com/gun-parts/ar-15-parts/ar-15-gas-system-parts/ to ensure everything aligns. Stop ignoring your gas system. Match the port location to the barrel length, control your dwell time, and your rifle will run smoother and last exponentially longer.