June 27, 2017
By Jeff Hoffman
Primers are composed of three basic components: a cup, the part that is visible in the cartridge; an anvil, a small tripod-shaped piece of metal; and an impact sensitive explosive. The cup contains the explosive primer mix sandwiched between the bottom of the cup and the anvil. Primers are designed to be struck in the center by the firing pin with a specified force in order to ignite the cartridge. When struck, the cup is dented inward, causing a shearing action on the explosive compound as the firing pin crushes it against the anvil in the primer. When that occurs, the primer detonates, and with incredible reliability.
"As you know, we build our primers to service a wide range of firearms. Our primers are built to meet SAAMI (Sporting Arms and Ammunition Manufacturer's Institute) specifications as determined by the SAAMI-specified drop test," said Glen Weeks of Winchester ammunition.
"Based on this test, statistics will tell you that our primers are 99.9997 percent reliable. That means that when struck with sufficient energy and properly centered, they will go off 99.9997 percent of the time. A lead styphnate primer is probably one of the oldest and most reliable devices on the planet."
SAAMI is an organization of industry members that sets standards for both firearms and ammunition. The standards are to ensure that the ammunition and firearms are compatible and work reliably and safely together.
Firearms and ammunition must have a set of minimum and maximum dimensions. These are set so that the largest allowable dimension ammunition will chamber and function even in the smallest dimension chamber. They also ensure that the maximum pressure developed by a cartridge will be within the limits of the firearm. SAAMI standards are also designed to assure that primers are sensitive enough to go off in a properly designed and chambered firearm in proper working condition.
They happen, but misfires due to a defective primer are incredibly rare. At Black Hills Ammunition, we test-fire over 100,000 rounds per year and sell millions of rounds to demanding customers. When we
encounter a misfire during testing or have one reported, we fully investigate it. Over the years, these investigations have shown that misfires due to defective primers are a very small percentage of misfires.
The point I am getting to is about statistics. Primers are not perfect — nothing man-made is — but they are reliable. Statistically, you may encounter a defective primer if you shoot enough. According to Weeks' primer data (99.9997 percent reliability), you might hit one every 300,000 rounds. That said, our experience at Black Hills does not indicate that high of a misfire rate.
We visually inspect each of the many millions of primers we get yearly from Winchester and other primer manufacturers. On average, we reject one primer out of 250,000, and most of those are due to minor defects or damage that would not effect reliability. The odds of encountering two misfires due to defective primers in one day are beyond remote. So, you ask, what can cause this situation? There are a number of possibilities.
CAUSES OF MISFIRES
The most common cause of misfire is referred to as "insufficient indent," which means the primer was not struck with sufficient force.
"But look at the indent," you say. "It was huge. I even hit it several times and it still didn't go!"
Primers are built with a very precise orientation of parts. If the primer is hit once and does not go off, its sensitivity is further decreased. The primer mix (called a pellet) may have cracked and moved out of the way. Remember, the pellet is intended to detonate when sheared between the cup and the anvil. If the first blow was insufficient, the second one is now trying to ignite a primer that was damaged by the first strike.
Sometimes the primer will go with a second hit, sometimes it won't. Hitting it again doesn't prove anything either way. It does, however, make the indent deeper. It also destroys the evidence necessary to do a thorough investigation. This is something to remember. If you have a misfire and want to investigate the cause, do not strike the round again. Save the round and its box, contact the manufacturer and return it for examination.
How can the weapon cause a misfire? One possibility is a damaged firing pin (bent or chipped tip). Another possibility is a weak firing pin spring. The spring may be dragging on the interior of the bolt body or impeded by grease in the bolt, especially in cold conditions. The pin could be too short.
Misfires can be caused by excessive headspace on the rifle. Headspace is the measurement from the slope of the shoulder to the base of the cartridge or the corresponding dimensions in the rifle chamber. Excessive chamber headspace allows the cartridge to be farther forward in the chamber, or the case might move forward when struck by the firing pin. These all can affect ignition reliability.
Improper loading techniques can also cause primers to perform poorly. For best sensitivity, the primer should be seated firmly to the bottom of the primer pocket, but not unduly crushed into place. The legs of the anvil are slightly compressed during the seating process (called cocking the primer). If the primer is not seated to the bottom of the pocket, the primer is not as sensitive because it moves forward slightly upon impact with the firing pin, thus cushioning the impact force. Conversely, if the primer is seated with too much force, it is farther from the firing pin.
If the case is undersize, it can move forward in the chamber upon impact just as if the chamber was too long. Contamination of the primer by moisture or oil can also desensitize the primer. These possible causes are not often seen in factory-loaded ammo due to quality control procedures and are more commonly found with handloaded ammunition.
Another common issue that can occur is the failure of the operator to completely close the bolt before firing. The bolt handle lifted even slightly can result in misfires. I discussed this with George Gardner of G.A. Precision.
"It's very common for the user to get caught up in the moment of a match or training exercise and blow one of the easiest things to do — close the bolt completely," Gardner said. "Speed is not always the best option. Smooth is faster. When the bolt is not fully closed, the firing pin cocking piece drops onto the camming surface of the bolt and not in the open notch that is there for clearance. This robs a lot of the energy of the falling firing pin."
Try this the next time you go to the range: Unload your rifle. Double check to make sure it is unloaded. Close the bolt, and then raise the bolt very slightly. Squeeze the trigger while watching the bolt handle. You can see the bolt handle snap closed. The energy to make that happen is being robbed from the firing pin velocity and energy normally used to strike the primer. Failure to completely close the bolt is easy to do, especially under speed, stress and position shooting. Normally we get away with it. Sometimes it induces a misfire.
I also discussed this with Charlie Milazzo, technical advisor to American Sniper Association. "Failure to completely close the bolt is probably the single most common cause of all misfires in sniper rifles," he said.
As an ammunition manufacturer, I have the obligation to investigate misfires. One common story I hear is: "I know it can't be anything wrong with my rifle because I have never had a misfire with brand XYZ." If the rifle works with one brand, why can't the shooter expect it to work with all brands?
All U.S. primers are made to an industry specification, but that specification is a range, which means that some brands are more sensitive than others. This is not because of quality differences; it is by design. Everything in life is a compromise. Primer manufacturers make choices. My company uses Winchester primers. I have discussed this topic at length with Winchester. They could make their primers more sensitive, but that increases the possibility of slam-fires. A slam-fire is the premature detonation of the round in the firearm from impact by the bolt or, in some cases, an inadequately restrained firing pin. This can happen most frequently during the feed process and prior to the round being fully chambered in a semiautomatic or automatic firearm.
Consequences of a slam-fire are generally more serious than a misfire from a light strike (60,000 psi of escaping gases from the breech is not a good thing).
Military primers are made with an even thicker cup than the one used in commercial primers specifically for this reason. Think about that. If a sniper rifle will work only with its favorite brand of commercial ammunition, what would happen if you fed it Mil-Spec ammunition?
Winchester could also make primers more sensitive by using a different anvil configuration, but while this increases sensitivity to centered primer hits, it decreases sensitivity to slightly off-center hits. They chose to stay with safe, reliable performance in a wide range of circumstances. Manufacturers must load for everyone's rifle. Most snipers shoot bolt-action rifles. However, ammunition must be safe in every rifle out there. They can't make primers more sensitive and label them "Not for use in semiautomatic rifles." Besides, do not confuse sensitivity with reliability. Primers are incredibly reliable when struck with industry specified force.
So what does it mean when the rifle experiences misfires with one brand of ammunition/primers but not with others? My opinion, and Milazzo's, is that such a rifle is marginal in performance. The sniper using such a rifle is betting against fate that the rifle will work when he needs it. I suggest they resolve the light strike issue with the weapon and use whatever brand of primer and ammunition they prefer.
The most common culprit of contaminated ammo is light penetrating oil. Oil is an extremely effective desensitizer of primers, which explains why most ammo has lacquer-sealed primers. Moisture can also
damage primers, but not to the extent of light penetrating oil. Brief exposure to moisture in the field is well tolerated. Long-term exposure, however, such as storage in damp conditions or pressurized water exposure (diving with your ammo) can damage ammo, but this is a military concern and not normally an issue for law enforcement.
Misfires are not that uncommon. However, misfires due to a defective primer are rare. So when you have a misfire, don't strike the primer twice (for examination purposes), but do investigate and fix the problem so it does not reoccur at a very bad time. Do not just chalk it up to a "bad round" and move on.
Keep in mind that the most common cause of misfires in a bolt-action rifle is operator error, specifically the failure to completely close the bolt. Fortunately, this problem can be corrected through awareness, training and diligence on the part of the operator.
Any sniper, team leader or commander who accepts misfires and does nothing to investigate and fix the problem is making a mistake that can have severe consequences down the road. We train hard to be capable of doing our job when called upon. It is not unreasonable to expect the same level of reliable performance from our rifle and ammunition.