Robert Silvers, who is head of R&D at AAC and inventor of the AAC Blackout flash suppressor, explains how flash suppressors work, and how his thorough understanding of the science involved shaped the AAC Blackout. Reprinted with his permission.
When designing (the AAC Blackout), I read a bunch of patents and did not find many instances of inventors knowing why flash formed or why their design works. Because of this, they often added in features which did more harm than good. I kept the Blackout efficient by not making those mistakes.
There are three main types of muzzle flash: Primary, intermediate, and secondary. Additionally, there may be an “afterburner” effect. Mechanical flash suppressors, such as the AAC Blackout, have little opportunity to control the part called primary flash (the sparks you sometimes see) because that is powder burning within the barrel. But we can prevent the external flare up which we most associate with muzzle flash – that is the secondary and afterburner effect flash.
In order for flash to occur, fuel, oxidizer, and a source of ignition must be present. The fuel is contained in the smokeless propellant. Ignition comes from heat â but only if an oxidizer is available from either the propellant itself or the surrounding atmosphere.
Typical smokeless propellants contain more fuel than needed to balance with the oxidizer. The mixture is run ‘rich’ to keep flame temperatures lower so as to reduce barrel throat erosion. This imbalance, while reducing the temperature of the mixture, has a side effect of expelling excess unburned fuel from the barrel. This particulate is what makes secondary flash possible. And as said, if you reduce it – the barrel will burn out sooner.
What is the source of heat for the ignition? Surprisingly, not the burning of the powder – but rather it has to do with the nature of supersonic gas flow. When the bullet exits the muzzle, the discharge of expanding gas is moving at supersonic speeds. The speed of the gas is faster than the bullet itself so it will be supersonic even if the bullet is moving slower than the speed of sound. Because the gas is under such high pressure, it is ‘under expanded’ when it is released to the environment. This pocket of particles and gas is contained within the shell of the external blast wave and as it expands, it cools. The external environment is pushing back, and the shock wave that forms will reflect this discharge back onto itself into a reverse shock wave known as a Mach disk.
These formations occur any time a flow exists a nozzle at supersonic speeds and at a pressure that is higher than that of the external atmosphere and are sometimes visible behind certain high performance aircraft. It is at this location that the supersonic flow changes to subsonic. When the shock wave passes through the Mach disk, the sudden deceleration and resulting compression greatly raises temperatures and can ignite the remaining fuel, provided there is oxidizer available.
If the oxidizer used during ignition is provided by the propellant, the result is a combustion known as secondary muzzle flash. If the oxidizer is provided by the surrounding atmosphere, it is more precisely described as an afterburning effect. In either case, the result is what we think of as muzzle flash.
The two main ways to suppress flash are by chemical or mechanical means. Flash retardants may be mixed into smokeless propellant to reduce the potential for flash. Generally alkali salts, 0.5 to 5.0 % by weight are used. Flash suppressants are usually not in propellants because they degrade the performance and increase smoke. Military customers often request the chemical additives, but the amount used, in consideration for the negative effects, is likely chosen to be effective only for typical barrel lengths and only most of the time. This means that shorter than normal barrels may find that typical mechanical flash suppressors (such as the A2) â even when combined with chemical flash retardants â are not sufficient to eliminate visible flash.
The AAC Blackout works to reduce pressures and temperatures in the gun muzzle flow field and hence there is also a reduction of strength of the resulting Mach disk. It does this by dividing the expanding flow into several weaker streams. Because of the weaker Mach disk, there is less sudden compression in a concentrated area of the gases as they go from supersonic to subsonic, and so the gas and particulate temperature will stay below the level required to initiate secondary combustion or afterburning.