During the early part of its career, the M16 had a reputation for poor reliability and a malfunction rate of two per 1000 rounds fired. The M16's action works by passing high pressure propellant gasses tapped from the barrel down a tube and into the carrier group within the upper receiver, and is commonly referred to as a "direct impingement gas system". The gas goes from the gas tube, through the bolt carrier key, and into the inside of the carrier where it expands in a donut shaped gas cylinder. Because the bolt is prevented from moving forward by the barrel, the carrier is driven to the rear by the expanding gases and thus converts the energy of the gas to movement of the rifle's parts. The back part of the bolt forms a piston head and the cavity in the bolt carrier is the piston sleeve. It is more correct to call it an "internal piston" system. " This design is much lighter and more compact than a gas-piston design. However, this design requires that combustion byproducts from the discharged cartridge be blown into the receiver as well. This accumulating carbon and vaporized metal build-up within the receiver and bolt-carrier negatively affects reliability and necessitates more intensive maintenance on the part of the individual soldier. The channeling of gasses into the bolt carrier during operation increases the amount of heat that is deposited in the receiver while firing the M16 and causes essential lubricant to be "burned off". This requires frequent and generous applications of appropriate lubricant. Lack of proper lubrication is the most common source of weapon stoppages or jams.