Gas Block
Component on the barrel that taps gas to cycle the action of a gas-operated firearm.
The gas block sits over a small port drilled in the barrel. When the bullet passes the port, expanding gas is diverted through the gas block and into the gas tube (on AR rifles) or the gas piston (on piston-driven designs), pushing the bolt carrier rearward to cycle the action. Adjustable gas blocks let the shooter tune the system.