Blowback is one particular system in which an automatic firearm may operate. The other systems are recoil-actuated, gas-actuated, gatling and chain.
In the blowback system there is no positive lock between the bolt and the barrel. The mass of the bolt and force of its recoil spring act to keep the breech closed. The expanding gases from the fired round overcome this inertia and "blow back" the breech. The breech must be kept closed until the round has left the barrel and gas pressures have subsided. The weight of the bolt is the major factor in determining this, and to remain practical this system is only really useful for weapons using relatively low pressure pistol rounds. Blowback operation is typically found only on pistols and submachine guns.
Blowback can also refer to the combination of gasses, dirt, and debris (unburnt powder, metal shavings) that most firearms produce upon firing. This can cause great irritation to the eyes and many ranges or organizations suggest or require the use of safety glasses when firing pistols.
More generally, the term blowback is used for any negative effect one suffers from one's own weapons.
