Problem

The very operating principle of a firearm is based on the thrust of the projectile by a pressurized gas. This gas is obtained by the combustion of a propellant charge inside the closed barrel, at the rear by a breech, while the seal between the barrel and the projectile is maintained either by direct contact between the barrel and the projectile, or by means of a belt, or through a sabot in the case of sub-caliber ammunition.

The performance of a weapon is often equated with the maximum kinetic energy given to the projectile as it leaves the barrel. Regardless of the projectile’s mass or desired muzzle velocity, to maximize the weapon’s performance, the pressure at the projectile’s base must be as high as possible for as long as possible, which implies maximum pressure throughout the barrel. Thus, increasing the performance of a weapon necessarily means subjecting the barrel to greater mechanical stress along its entire length.

State of the Art

The maximum operating pressure of a weapon is a parameter defined during ammunition design primarily by the strength of the standard barrel to be used. During this process, a number of parameters are fixed more or less definitively in the form of standards, so as to guarantee interoperability between weapons and ammunition of the same caliber, in complete safety for the operator.
In the case of high muzzle velocity firearms, the thickness of the barrel wall at the breech is often greater than the internal diameter of the barrel, which means that the maximum working pressure is directly linked to the elastic limit of the barrel material. Thus, an ammunition designer has only a limited number of solutions to improve the ammunition performance :

Increase the duration of maximum thrust: by lengthening the barrel and increasing the volume of the case to increase the propellant charge, but above all by adopting much more complex powder
geometries so that maximum pressure is maintained for longer despite the increased volume available for the propellant gases as the projectile advances through the barrel.
Increase the bearing surface of this maximum pressure by increasing the caliber of the weapon and adopting a sub-calibration device.

While the quality of the steels used to manufacture firearms has always been a driving force in improving the performance of these weapons, it has to be admitted that a performance peak was reached when the steels developed for nuclear programs were made available to the arms industry in the 1970s-1980s.

Proposed solution

To reduce the stress on the gun during firing, it is imperative to transform the energy distribution within the gun, propellant gas and projectile assembly, so as to reduce the proportion of thermal energy from the propellant gases within the gun, which is responsible for the pressure on the gun walls. The proposed solution consists in maximizing the velocity of the gases in the barrel axis as early as possible in the barrel, so as to transform part of the thermal energy of the propulsion gases into kinetic energy.
To achieve this, it is necessary to separate a combustion chamber, in which the high pressure required to maintain combustion prevails, from the barrel, in which the propellant gases push against the projectile, mainly the projectile itself. The separator consists of a Laval nozzle which, once primed, allows the propellant gases to undergo a significant expansion before being recompressed behind the projectile.

This solution offers the following advantages:
Once the nozzle is primed, the pressure distribution in the barrel passes through a minimum at the nozzle outlet and a progressive compression along the barrel, so that the maximum pressure in the barrel is at the projectile base, as long as the nozzle is primed.
The gases leaving the nozzle, once primed, have a higher velocity than that of the projectile, making propulsion of high muzzle velocity projectiles more efficient.
A large proportion of the solid propellant charge can be trapped in the combustion chamber, whose volume remains constant during firing. This simplifies the powder geometry required to achieve
constant pressure over time, since it is no longer necessary to compensate for an increase in volume by increasing the combustion surface. It also minimizes unburnt emissions, since the pressure in the combustion chamber is fixed by the cross-section of the nozzle throat.

Calculated Performance

In addition to reducing the pressure applied to the barrel walls, the implementation of this innovation translates in theoretical results into a second peak in projectile acceleration at muzzle initiation. This implies a more efficient use of the entire barrel, and therefore a higher muzzle velocity for the same barrel length.