A break-action firearm is one whose barrels are hinged, and rotate perpendicular to the bore axis to expose the breech and allow loading and unloading of ammunition. A separate operation may be required for the cocking of a hammer to fire the new round. Break open actions are universal in double-barrelled shotguns, double-barrelled rifles and combination guns, and are also common in single shot rifles, pistols, and shotguns, and can also be found in flare guns, grenade launchers and some older revolver designs. They are also known as break-open or break top actions.
A substantial hinge pin joins the two parts of the rifle or shotgun; the stock with its firing mechanism and the fore-piece and barrel, which hold the round to be fired. In some cases the hinging pin may be easily removable, allowing the two portions of the weapon to be compactly and safely stored. In other cases the hinge will consist of a hook over a pin; releasing an auxiliary latch will allow sufficient travel to allow the hinge to be unhooked.
A latch is operated to release the two parts of the weapon, allowing the breech to be exposed. A shell is inserted into the breech (up to two for a double barreled shotgun and six to eight for a revolver), and the mechanism is closed and latched. When the operator has positioned the weapon downrange the hammer is pulled back and latched. The weapon is now ready to fire, done by squeezing the trigger.
After firing the round(s), the break action is unlached and the barrel and forearm are allowed to fall forward. This will cause an extractor catch to remove the spent shell, which is allowed to fall to the ground. The weapon is now ready for a new cycle.
Since the cartridge extractor or ejector is built into the barrel assembly on break open actions, the breech face is simple a flat plate with a hole for the firing pin to project. This makes break open actions ideal for interchangeable barrel firearms, such as the popular Thompson Center Arms Contender and Encore pistols. The simplicity of the break open design, especially external hammer types, also reduces the cost of manufacture. There are a number of companies, such as H & R Firearms, producing break open rifles at far lower costs than comparable bolt action rifles.
Another advantage of the break open action is the ability to insert very long objects into the chamber. This allows cartridges of a length impractical in most other designs, as well as easy use of caliber conversion sleeves. It is common to find double barrel shotguns available with conversion sleeves for smaller gauges, allowing the same gun to be used with, for example, 12, 20, 28, and .410 gauge shells
The break open design is best suited for non-repeating firearms. To get multiple shots requires either multiple barrels or a revolver cylinder; while this is fairly simple for shotguns, with double-barreled shotguns quite common and even four-shot designs like the FAMARS Rombo available, double rifles require very precise alignment of the barrels so they shoot to the same point of aim. Modern double rifles are very expensive, and designed for short range use; the barrels are often regulated for ranges under 100 yards (90 m) for use against dangerous game.
Wear in the mechanism is focused upon the small contact area of the latch, and the breech is difficult to seal properly once the latch wears. In some firearms, such as the Thompson Center guns, the latch was a removeable part that could be replaced when worn. Designs without a replaceable latch could be fixed by building up the worn part with a welder, then filing back to shape.
Break open actions are also not as inherently strong as other action types. The action is generally held closed by a single locking lug, usually below the barrel in a single shot or between the barrels of a double barrelled gun. This is not as strong as the bolt action design, which generally uses multiple locking lugs around the perimeter of the bolt to provide an even distribution of the forces of firing. Since many break open rifles, such as the inexpensive H&R models, are build on large frames originally meant for shotguns, the action is very heavily built. Still, belted magnum cartridges such as .300 Winchester Magnum are generally only found in the highest quality, very expensive break open rifles.
Another disadvantage of the break-open action is the low pressures it is capable of withstanding. The single locking lug is subjected to all the force from the firing of the cartridge, and the small area (compared to, say, a bolt action design) means that the total force (the product of pressure and cartridge head diameter) allowed for safe operation is limited. For shotguns, which operate at very low pressures, this is not an issue; the Thompson Center Contender, on the other hand, was limited to .30-30 Winchester level cartridges. To fire more powerful cartridges requires a much larger locking lug, such as that which Thomson Center used on the larger Encore model.
Break open designs work best with rimmed cartridges, which can use a solid extractor. Rimless cartridges require a spring-loaded extractor, which can slide out of the way for cartridge insertion, and spring back to engage the recessed rim. While these spring loaded extrators are found on even inexpensive models, the spring loaded extractor is not as strong as a solid extractor, and increases the chance of failure to extract.