November 17, 2020
When I was “CSI before it was cool,” we investigated every officer-involved shooting and were usually able to interview the officers. It helped us better understand armed police engagement, training, and, yes, ammunition.
Our terminal ballistics evaluation of police ammo using gelatin testing began with a series for the Dallas PD. It involved 15 kinds of .38 Special ammo that was available in the early 1970s. Our primitive photo-electric chronograph screens required us to set the range lighting to “fairly dark,” just enough for safe gunhandling and data recording.
One .38 Special load tested was, shall we say, “enlightening” for all the wrong reasons. I fired the first one and heard an expletive uttered behind me that I won’t repeat. My lab partner who was recording data was blinking from the bright yellow-white flash.
I was focused on the target so didn’t experience the flash as much, but I did notice more recoil. The load posted nearly 200 fps higher velocity than similar .38 Special ammo.
We photographed these flash effects in a completely dark room. Most of the images were black except for a small flash preserved in white. The flashy load was a flash bulb. The image showed all the room details, including legible box labels. When we expanded testing to magnum revolver cartridges, it was clear the .38 Special load was not alone. We had to ask ourselves if an issue we saw in the lab translated to a problem in the street. We soon had the chance to ask—with that same .38 Special load.
One evening after sundown, a man in a car fired on an officer through the driver’s window, forcing the officer to respond. The officer returned fire with three of those flashy .38 Specials. I interviewed the officer and asked about muzzle flash and recoil. “What flash? What recoil?” he answered. “All I remember is a hailstorm of glass headed my way that had a bullet in it somewhere!”
That response echoed through nearly every subsequent interview. I found only one where flash and blast temporarily disabled an officer, and that officer never fired.
Two officers approached a suspect, who pulled a gun. The officer who related the story dropped to a kneeling position while drawing his sidearm. His partner did a quick-draw and fired from the hip. That put a .357 Magnum revolver about a foot from the kneeling officer’s head. “Good thing my partner hit the guy,” the officer said, “because I couldn’t see or hear for about 15 minutes!”
Yes, there is a flash/blast effect, but it’s not as significant to the shooter as to the observer. Although this could be a useful side-effect of stress-fueled target fixation, reducing flash is always a good idea.
In the mid-1990s there was a renewed interest in developing better police ammunition, including reducing muzzle flash. The thermo-chemical origins of muzzle flash have long been understood. Tenney L. Davis discusses it in The Chemistry of Power & Explosives (John Wiley & Sons; New York; 1943) and mentions efforts to create low-flash propellants going back to the adoption of smokeless fuels.
The primary concern was for artillery pieces. Davis states that a 12-inch naval rifle can emit a flash that is up to 150 feet long. That’s one-quarter the length of old battleships that mounted them. Reflections on clouds could be seen for 30 miles.
Chemists like Davis always understood that irrespective of the size of the tube, flash is primarily a burning gas cloud rather than partially burned propellant particles. Today’s high-speed photography of small-arms flashes supports that, capturing billows of flame not a mass of burning particles. Typically, most glowing particles photographed are powdered aluminum from the primer.
Nitrocellulose propellants carry their own oxygen supply, but it is consumed in the initial deflagration. Flammable gases like hydrogen and carbon monoxide are pushed down the barrel and starved of oxygen. When these super-heated gases meet oxygen in the air in front of the muzzle, the gas cloud ignites.
Flash can be controlled by firearm alterations like flash hiders; modifying powder charge weights; or with chemistry, either in the form of additives or modifying the suite of explosives that form a propellant’s “base.”
Changes in charge weight work because one unit of burning propellant generates a predictable amount of gas. All other things equal, doubling the charge weight doubles the gas volume produced. Adjusting charge weights to reduce flash without losing velocity is tricky work. Lower weights may be ballistically efficient only with faster-burning propellants, and those may not give enough velocity to make specs.
Additives usually fall into a broad chemical family know as salts. Many of these, such as potassium chloride, are corrosive. Fortunately, flash suppression salts are usually in small quantities, below what corrosion requires. They gave good flash control by lowering the flame temperature but often exacted a small performance penalty. Still, most salts are hygroscopic (they absorb moisture), complicating the manufacturing process.
We all know about single-base and double-base propellants. Each “base” is a primary explosive: nitrocellulose in a single-base fuel and nitrocellulose plus nitroglycerin is a double-base product. As early as the 1930s, triple-base propellants were known, adding a third primary explosive. Davis describes a “cool explosive,” ammonium nitrate or nitroguanidine, for flash control. Ammonium nitrate generates smoke, so nitroguanidine got the nod for many flash-suppressed propellants.
The strangest flash effect I recall was while qualifying propellants to load .357 Sig Gold Dot factory ammo. One candidate produced little flash for four or five shots, then delivered a beachball-sized firestorm, then reverted to minimal flash for another few rounds before erupting again. This “off-on-off-on” sequence was absolutely predictable. From Davis and other references, we thought the only reasonable explanation was that the barrel was heating up incrementally, requiring several shots before enough heat was added to the system to super-heat the gas. Each big flash pulled the heat out of the system, and it took another four or five shots for it to build up again.