July 24, 2024
By Allan Jones
Firearms have always been tubes from which an energetic material—propellant—ejects some form of projectile. How that propellant was activated forced the evolution in firearm and ammo designs that we use today. All percussion priming relies on physical shock to force a violent chemical reaction, creating extreme heat and gas. Its forebear, flint-on-iron ignition (“flintlocks”), relied solely on mechanical friction and sparking to light off the charge. Flintlocks required an intermediate step to ensure proper ignition: fine-grained propellant in a shallow pan next to the barrel. Were the pan charge lost due to wind or leakage, there was no “boom” from your boomstick. In ignition history, percussion-sensitive chemical materials were lumped together as fulminates or fulminating compounds, from a late Latin word for “lightning.” Although many metals can be converted to fulminates, mercury fulminate, Hg(CNO)2 became the preferred percussion priming compound.
Obviously, mercury’s toxicity was little known 200 years ago. First described in scientific literature in 1800, mercury fulminate pellets became the heart of Rev. Forsythe’s “scent-bottle” priming system in 1807. By 1822 fulminate was held inside soft metal caps to eliminate flintlock pans. Caps slipped on a conical “nipple” on the breech that vented to the powder charge in the barrel. These arms were “caplocks.” Mercuric priming persisted well into the cartridge era—the U.S. military used it until 1898—but it was not perfect. The power of mercuric primers degraded with time, causing misfires and hangfires. Mercury residues do not affect steel but easily combine with copper alloys, making fired brass cases brittle. The replacement nonmercuric mix was based on potassium chlorate, KClO3, and lead thiocyanate, Pb(SCN)2. These are the “chlorate” primers cited in many reloading texts. These were reliable and stable, but one of the combustion byproducts of KClO3 is potassium chloride, KCl, commonly used as a sodium-free substitute for table salt. It will rust steel quickly if not removed from gun barrels using water-based methods. These were corrosive but nonmercuric. U.S. arsenals did not finally abandon chlorate priming for service ammo until the early 1950s. However, because it had performed so well for so long, the U.S. FA-70 chlorate primer was retained for use in .30-06 military match ammo well past the primer changes to service ammo. However, one cartridge featured noncorrosive, nonmercuric priming years before other U.S. service cartridges were converted.
The U.S. .30 Carbine cartridge was developed around 1940 and always had noncorrosive priming. Why? Gen. Julian Hatcher explains that .30 Carbine ammo was always intended to be loaded by commercial ammomakers to military specifications (Hatcher’s Notebook, 3rd Ed., p. 487). While preparing those specs, Ordnance staffer Col. E.H. Harrison examined the captive-piston design at the heart of the M1 Carbine’s gas system and thought it was highly vulnerable to corrosion. Harrison’s final specifications stated primers loaded for .30 Carbine must be noncorrosive. Although he did not specify a particular formula, by then all commercial U.S. primers featured noncorrosive compounds virtually identical to what we use today. Yes, all military-issue .30 Carbine ammo loaded in the U.S. has noncorrosive primers. That does not mean .30 Carbine ammo loaded in other countries is noncorrosive. Some is highly corrosive. Been there, done that, got the rust-stained T-shirt. Commercial ammomakers had established nonmercuric, noncorrosive priming as the new standard in the 1920s. The usual initiator was lead styphnate, C6HN3O8Pb, explored as early as 1874. The corrosive chlorate had to go. Efforts toward better primers took different paths in different countries, but most conventional styphnate primers, then and now, share the same suite of ingredients. The core components in most modern noncorrosive, nonmercuric primers are an initiator, lead styphnate; an oxidizer, barium nitrate; and a fuel, antimony sulfide. Any variation in the ratio of these among primer brands is usually to maximize each company’s “house blend” to be tuned to the metal-parts configuration—cup and anvil characteristics—of their primers.
Refinements led to adding smaller amounts of other things to help that trio work better. Atomized aluminum powder may be added as a secondary fuel that projects incandescent particles into the propellant charge. Another percussion-sensitive material, tetracene, is common as a “sensitizer.” Although it’s much weaker than styphnate, its activation energy under impact is somewhat lower. It serves as a “booster charge” to ensure styphnate’s activation under a wide range of conditions. Rimfire priming mixes may contain finely ground glass. This is a frictionator, intended to make ignition more reliable in weak “pinch-rim” systems, and adds incandescence. This was de rigueur until the 1980s when air quality concerns—specifically airborne lead—arose in indoor shooting facilities. By combining bullets having clad or protected bases with primers having nonlead initiators, the level of lead at the firing point could be reduced to what one published law enforcement study described as “residual amounts left in the firearm from prior leaded-ammo use.”
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The nonlead initiator used for the first modern lead-free priming was not a new compound, but it was a mouthful. Diazodinitrophenol (DDNP) was described by the German chemist Griess as early as 1858 and further studied in the 1930s. In the mid-1980s, it became the preferred reduced-toxicity initiator of the day. Early DDNP primers were slightly less sensitive than styphnate primers. They could also be picky about the kinds of propellants they had to ignite; many did not light off spherical-type propellants very well. With the lead gone, researchers focused on the barium and antimony as well. Oxides of “lighter” metals like zinc replaced barium nitrate. Some efforts substituted finely ground smokeless propellant instead of antimony sulfide as a fuel. Work continues to make such primers as ballistically flexible as styphnate, but my “old retired guy” status does not give me the access to today’s proprietary research I once enjoyed. Still, it’s apparent that the need for reduced-toxicity priming is not going away.
