Firearms have historically been designed with a single-bore barrel. This has been the case due in part to technical difficulty of boring a long straight hole through a piece of hard metal, and the problem of cutting or impressing a rifling pattern on the interior of the bore without affecting the trueness of the barrel and hence its accuracy potential. The number of variables involved in those processes is so large as to render the specialized field of barrel making as much an art as a science. That situational difficulty is compounded to near impossibility in the attempt to bore a second hole in the same metal piece, if both bores are to be straight and accurately aligned. For those reasons, accurate multi-bore barrels have never been successfully manufactured.
All successful battle rifle designs, from the time of the smooth-bore matchlock until the present day, have been of the single-bore, single-barrel type. These firearms have reached a high degree of refinement after centuries of development, and share the use of cartridge ammunition, which is a useful solution to the problem of how to quickly reload a barrel for firing. Cartridge ammunition is likewise highly refined after a long development. The modern battle rifle has resulted from the combination of a single barrel designed to be loaded with cartridge ammunition from the breech, a mechanism or “action” that inserts and replaces cartridges into the barrel, and a magazine that contains numerous cartridges.
Existing rapid-fire mechanisms require considerable energy to function. A heavy steel bolt must be quickly moved against a powerful spring. Relatively long and heavy cartridges must be inserted into deep chambers, and then rapidly removed. The total motion of the bolt for each shot can be 6-8 inches or more. At the same time, a firing hammer must be cocked against its own heavy spring, and then released by another linkage. All this back and forth consumes so much energy that the modern battle rifle emerged only after the invention of the gas operating system. The gas system provides the needed energy by using high pressure gas from the bore to move a piston or other mechanism which then moves the action. While ingenious, the gas action has its own problems.
An action powered by the high pressure and extremely hot gas produced by the propellant has the force needed to power a complex action. However, that same gas is contaminated with metal vapors and particulate matter that may foul the delicate inner workings of the action. This can result in jamming and other undesirable failures that require frequent field maintenance. Furthermore, the need to clean a weapon in the heat of battle may have fatal consequences.
Cartridge ammunition requires a chamber machined into the bore at the breech end of the barrel where the interior dimensions of the chamber closely match the exterior dimensions of the cartridge type chosen for the weapon. The weapon designer must select a cartridge type with ballistic characteristics approximating the desired performance of the intended weapon design, considering both the trajectory and terminal ballistics. The weapon must then be designed to accommodate the exact physical dimensions of the cartridge, including the standardized (SAMMI) maximum pressures created by the selected cartridge. This situation in turn imposes a set of parameters under the weapon design, such as general size and weight, material selection, magazine type, action type, barrel length, firing rate and magazine capacity. These factors, and others, have led to a design convergence toward a popular basic layout of a single-bore barrel with a gas-operated action, a box magazine containing 30 cartridges or so, and .22 to .30 caliber cartridges of approximately 3,000 FPS velocity. These specifications are mostly a result of all the compromises required to achieve a practical design.
The role of cartridge ammunition in the functioning of the weapon is important in defining the limitations of a single bore design. The cartridge case not only contains propellant and other components, it also performs the critical function of sealing the breech during firing. When the propellant is ignited and pressure builds within the cartridge case, its walls are forced outward against the interior of the chamber and form an adequate seal as long as the pressure is sufficient to keep the case expanded. There is a period at the beginning and another at the end of the propellant burning cycle when the pressure is elevated but insufficient to form or maintain the seal. This results in hot gases flowing through the action and consequent fouling. This is not a minor detail because these weapons function in a sequential progression with each step dependent upon the successful completion of the previous step. Any failure in any step brings the entire process to a halt until the cause is ascertained and corrected.
A rate of fire adequate for combat consumes significant quantities of ammunition and can quickly overheat the weapon, which can result in jamming. The effective rate of sustained fire is a very important measurement of the combat capability of a weapon in practical use. A rate of fire restricted to avoid overheating or including cooling periods may be considered to be the effective rate of fire, which over a sustained period of operation will always be less than the maximum cyclic rate of the weapon (and in most cases considerably less). Because the effective rate of sustained fire is always less than the maximum cyclic rate of the weapon, the buildup of heat is the limiting factor in combat capability. The overheating problem is an unavoidable consequence of the basic design of the single-bore cartridge ammunition weapon type, and the reasons are straightforward. Much of the intense heat produced by the combustion of the propellant passes through the cartridge case and is absorbed by the walls of the chamber. Importantly, the heat is generated on the interior of chamber and bore and must be conducted through the heavy steel walls of the chamber and barrel before it can escape. Steel is a relatively poor conductor of heat and interior surfaces can overheat before significant exterior cooling can occur.
A rapid rate of fire adds heat much more quickly than can be dissipated by conduction or convection, raising the temperature of the chamber walls. The chamber can become hot enough to ignite fresh cartridges upon entry or soon after. The pre-ignition of the cartridge (also known as “cook offs”) can result in cartridge feeding problems, and unintentional discharge of the weapon. With some designs, especially high cyclic-rate types, cook-offs can occur in as few as 150 rounds.
Overheating in ordinary single bore weapons is a limiting factor and an unsolved problem. The situation remains because it is the inevitable result of the basic design. For example, friction from the projectile and hot gas flow through the bore following discharge can add more heat to the barrel. The extreme heat generated within the cartridge upon firing is absorbed by the chamber then conducted to the rest of the barrel. The faster the firing rate and the more powerful the cartridge, the worse the situation becomes. The problem is acute in the chamber where most of the heat is concentrated. The chamber walls must be extra thick to maintain integrity when hot, and active cooling is not effective when heat is added more rapidly than it can be conducted away. Further, the size, weight and complexity penalties of active cooling are not worth the results for light arms. Moreover, the high heat loads in the chamber of a single bore, sequential feeding cartridge ammunition firearm inevitably affects the functioning of the action. A chamber can become hot enough for the softer metal of a cartridge case to melt and adhere to the chamber wall, jamming the firearm.
In the attempt to improve the effective rate of sustained fire and address some of those issues, multi-barrel firearms have been designed and built by Gatling, General Electric and others. A set of parallel barrels with conventional integral chambers, fastened together and rotated (in modern designs by an electric motor) around a central axis, are fed with cartridge ammunition by a complex mechanism. Each barrel fires in its turn and not again until all the others have fired, thus dividing the duty cycle of each barrel by the number of barrels. The mechanism performs the loading, firing, and unloading operations in different barrels simultaneously as the set rotates. Misfires or defective ammunition can process through the system normally and not cause a stoppage. Such an arrangement improves the rate of sustained fire by integrating the firepower and ammunition capacity of several automatic firearms together into a single machine. Also integrated are much of the size, weight and complexity of those several firearms, a large heavy magazine, as well as the additional weight and complexity of the electric drive and control systems and their associated power supply.
A multi-bore firearm, with several bores within a single barrel, could potentially exhibit many of advantages of a multi-barrel design, while reducing the size, weight and complexity disadvantages. Moreover, a multi-bore firearm with a single, fixed barrel containing bores that are precisely and permanently aligned to one another would eliminate accuracy challenges arising from the difficulty of achieving and maintaining he alignment of multiple moving barrels to each other and to the gunsights; from non-uniform warpage of the various use-heated barrels; from the centripetal forces acting on the barrels at their mounting points in a direction perpendicular to their axes; and from the angular momentum conserved by a projectile exiting a rotating system. Multi-barrel systems are considered very accurate if the projectile dispersion angle is in the range of 5-8 mils, while a multi-bore system has shown a dispersion angle of <1 mil in field testing of a non-optimized prototype.
A firing pin and miniature electromagnetic striker for each charge with overall electronic fire control eliminates the need for a heavy and complex mechanical firing system, while the use of charge blocks eliminates the need for cartridge ammunition and the necessary integral chambers and heavy reciprocating action. Hot charge blocks are ejected once exhausted, removing excess heat from the firearm. The total heat load of the barrel is divided among the multiple bores, reducing wear and facilitating cooling. Without integral chambers or the need to load cartridge ammunition, barrel heat does not affect the function of a charge block firearm, preventing cook-offs and allowing for near-continuous operation.
The energy required to activate the miniature electromagnetic strikers, camshaft actuator and electronic fire control circuits is low enough to permit the use of a lightweight onboard power supply sufficient to allow extended operations, many thousands of discharges and energy to operate various electronic firearm accessories.
Cartridge ammunition must be loaded into magazines before it can be used in self-loading firearms. Long term storage of cartridges in magazines is not recommended as this can weaken the magazine spring, and allow the accumulation of foreign matter in the magazine, which cannot be effectively sealed. In order to have the ability to quickly reload the firearm, an operator typically pre-loads by hand numerous magazines to carry along with the firearm. Many also carry a container of loose cartridges to reload the magazines, if necessary. To exchange an exhausted or partly exhausted magazine, an operator must handle both if he wishes to reload the ejected magazine later. Charge block ammunition does not require a magazine for transportation or storage. Charge blocks can easily snap together to form stacks that may be carried as is. The magazine can remain attached to the firearm and be refilled at any point by retracting the load knob and inserting fresh charge blocks through the ejection port. An empty or partially empty magazine can be quickly refilled with a pre-assembled stack of the correct size; if a partial refill, any extras can be snapped off. Individual charge blocks may be loaded by the same method. Release the load knob and the firearm is in the ready condition. Charge blocks are sealed units and may be transported or stored indefinitely either individually or in stacks.