Centerfire Ammunition - Primer Chemistry

Primer Chemistry

Primer manufacture and insertion is the most dangerous part of small arms ammunition production. Sensitive priming compounds have claimed many lives including the founder of the famous British Eley ammunition firm. Modern commercial operations use protective shielding between operators and manufacturing equipment.

Early primers used the same mercury fulminate used in 19th century percussion caps. Black powder could be effectively ignited by hot mercury released upon decomposition. Disadvantages of mercuric primers became evident with smokeless powder loadings. Mercury fulminate slowly decomposed in storage until the remaining energy was insufficient for reliable ignition. Decreased ignition energy with age had not been recognized as a problem with black powder loadings because black powder could be ignited by as little energy as a static electricity discharge. Smokeless powder often required more thermal energy for ignition. Misfires and hang fires became common as the remaining priming compound sputtered in old primers. A misfire would result if the priming compound either failed to react to the firing pin fall or extinguished prior to igniting the powder charge. A hang fire is a perceptible delay between the fall of the firing pin and discharge of the firearm. In extreme cases, the delay might be sufficient to be interpreted as a misfire, and the cartridge could fire as the action was being opened or the firearm pointed in an inappropriate direction.

Incandescent particles were found most effective for igniting smokeless powder after the primary explosive gasses had heated the powder grains. Artillery charges frequently included a smaller quantity of black powder to be ignited by the primer, so incandescent potassium carbonate would spread fire through the smokeless powder. Potassium chlorate was added to mercury fulminate priming mixtures so incandescent potassium chloride would have a similar effect in small arms cartridges.

Priming mixtures containing mercury fulminate leave metallic mercury in the bore and empty cartridge case after firing. The mercury was largely absorbed in the smokey fouling with black powder loads. Mercury coated the interior of brass cases with smokeless powder loads, and the higher pressures of smokeless powder charges forced the mercury into grain boundaries between brass crystals where it formed zinc and copper amalgams weakening the case so it became unsuitable for reloading. The United States Army discontinued use of mercuric priming mixtures in 1898 to allow arsenal reloading of fired cases during peacetime. Frankford Arsenal FA-70 primers used potassium chlorate as an oxidizer for lead(II) thiocyanate, to increase the sensitivity of potassium chlorate, and antimony trisulfide, as an abrasive, with minor amounts of trinitrotoluene. These corrosive primers leave a residue of potassium chloride salt in the bore after a cartridge is fired. These hygroscopic salt crystals will hold moisture from a humid atmosphere and cause rusting. These corrosive primers can cause serious damage to the gun unless the barrel and action are cleaned carefully after firing.

Civilian ammunition manufacturers began offering non-corrosive primers in the 1920s, but most military ammunition continued to use corrosive priming mixtures of established reliability. The various proprietary priming formulations used by different manufacturers produced some significantly different ignition properties until the United States issued military specifications for non-corrosive primers for 7.62x51mm NATO cartridge production. The PA-101 primers developed at Picatinny Arsenal used about 50% lead styphnate with lesser amounts of barium nitrate, antimony trisulfide, powdered aluminum and tetracene. Most United States manufacturers adopted the PA-101 military standard for their civilian production of Boxer primers. Manufacturers subsequently offered more powerful magnum primers for uniform ignition of civilian long-range or big-game cartridges with significantly greater powder capacity than required for standard infantry weapons.

Other explosives used in primers can include lead azide, potassium perchlorate, or diazodinitrophenol (DDNP). New on the market in the late 1990s are lead-free primers, which address concerns over the lead and other heavy-metal compounds found in older primers. The heavy metals, while small in quantity, are released in the form of a very fine soot. Some indoor firing ranges are moving to ban primers containing heavy metals due to their toxicity. Lead-free primers were originally less sensitive and had a greater moisture sensitivity and correspondingly shorter shelf life than normal noncorrosive primers. Since their introduction, lead-free primers have become equal in performance to lead-based primers, and are gradually gaining popularity.

Military-surplus ammunition often uses inexpensive corrosive or slightly-corrosive Berdan primers because they work reliably under severe conditions, whereas modern Boxer primers are almost always non-corrosive and non-mercuric. Determination of corrosive or non-corrosive characteristics based on the primer type should consider these final headstamp dates of corrosive ammunition production:

• .45 ACP: FA 54, FCC 53, RA 52, TW 53, WCC 52, WRA 54
• .30-06 Springfield: FA 56, LC 52, RA 51, SL 52, TW 52, WCC 51, WRA 54
• FN 57

For more detailed information on identifying USGI corrosive and non-corrosive ammunition based on cartridge headstamp, see Corrosive Primer Redux by M.E. Podany, ALGC. This article refers to The American Rifleman, "Beginners Digest: Nonmercuric, Noncorrosive Primers", pp. 34–36, January 1961.