Cycling operation refers to the mechanism by which a semi- or fully automatic firearm (or indeed a heavy weapon or piece of ordnance) cycles its action. There are several types of cycling operation, depending on how exactly the weapon uses the side effects of firing to cycle the action, of which the four most common are:
- Blow-back operation: Probably the simplest mechanism - the gases from the exploding propellant bear directly on the face of the bolt, pushing it back against a delaying spring in a simple "equal and opposite" reaction to the firing of the round. The bolt is blown back down the length of the action and then comes forward again under the power of the return spring, chambering the next round as it goes. This is an easy mechanism for low powered cartridges and is common in submachineguns, but tends to struggle with more powerful rounds and needs complicated engineering to ensure that the majority of the propellant energy goes towards putting the round out of the barrel rather than leaking into the receiver. Can also be quite violent and prone to tearing cartridge cases when used with high-powered ammunition. Sub types are divided on how blow back is delayed and for how long.
- Blow-forward operation: A rare and unusual mechanism, similar to blowback except that the bolt is fixed and the chamber and barrel are driven forwards off it by the shot. It has some advantages in controlling recoil, but is usually mechanically complex and involves the sort of excessive motion that is generally bad for accuracy. Not much favoured by weapons designers this mechanism is not really common enough to have sub-types.
- Recoil operation: This mechanism relies on the mechanical force of the weapon's recoil to cycle the action. Generally part of the weapon is held in place by the user whilst other parts recoil relative to it and cycle the action thereby. This is a common mechanism for high powered self loading pistols and some semi-automatic shotguns and very common indeed in artillery pieces. Many machine-guns are also recoil operated - not least the ancestor of the class the Maxim Gun. The downside to this form of operation is that it frequently requires quite violent motion from the working parts of the weapon which can be alarming and/or dangerous to the operator and impair accuracy. There are various sub types - including short, long and inertial recoil - depending on which parts recoil and how far.
- Gas operation: A gas operated weapon bleeds propellant gases from near the muzzle and uses them to drive a piston which cycles the action. This is a common mechanism for self loading rifles and assault rifles as it provides enough delay after each shot to allow chamber pressure to drop before unlocking the bolt, thus helping to cope with the demands of high powered ammunition. This comes at the cost of added mechanical complexity and cost of manufacture. Sub-types depend on whether the piston is attached to the bolt carrier or not, although this mechanism also includes systems by which the gas is piped back to drive the bolt directly without a piston and those in which the piston is driven by gases trapped beyond the muzzle. Both of these latter types lead to extensive fouling of the action with propellant residues and are held in great disfavour.
A firearm that lacks a cycling operation is either single action or possessed of some more esoteric form of operation - a driven action like that of a chaingun or something like the metal storm action are possibilities.