The term “slam-fire” is commonly associated with bolt action and military-pattern semiauto rifles (it could also apply to select-fire military rifles, but since most of us will never own such weapons, we’re talking semiauto for the vast majority of people). It means when the bolt goes forward, either when being closed vigorously by hand on a bolt action, or when coming forward under spring pressure in a semiauto, the gun fires without the trigger being pulled. Depending on whether or not the action is sufficiently closed when this happens, gun damage and even shooter injury are both possibilities.
Semiauto military-pattern rifles featuring free-floating firing pins, which have no spring pressure resisting forward inertia of the firing pin, are common culprits for slam-fires, especially when using reloaded ammunition. Instead of using a firing pin spring, they rely on making the firing pin itself large and heavy to retard its forward momentum, also milspec ammo for such rifles has primers that are quite hard.
Thankfully slam-fires are easily avoided. First, never NEVER ever ever ever fire any rifle, but especially a military-pattern weapon in .30-06 Springfield, .308 Winchester/7.62x51mm, 7.62x39mm, .223 Remington/5.56mm or .30 M1 Carbine, with pistol primers instead of rifle primers. Rifle primers feature cups considerably thicker and stronger and less ignition-sensitive than those on pistol primers. Sometimes, even on rifles that never slam-fire, if you rack a round out of the chamber without firing it, after it was chambered either by shooting a semiauto or cycling a bolt fast, you’ll see a minute firing pin strike where the tip of the firing pin has dimpled the hard rifle primer. With softer, more ignition-sensitive pistol primers, that cartridge might well have slam-fired.
Second, if your ammunition will be fired in a bolt action or semiauto military-style rifle, use Magnum primers. The term “Magnum” as it relates to primers scares a lot of people; they think these are super-high pressure primers, which will blow up their guns. What the term really means is they have thicker, harder cups than standard primers. Depending on the brand, and whether we’re talking pistol or rifle primers, Magnum primers might or might not also use a different priming compound, to ignite large quantities of slow-burning powder. In pistols, we’re talking primarily Magnum revolvers, thicker cups resist primer flow back into the firing pin hole when firing heavy loads; in rifles, thicker cups resist primer flow AND help avoid slam-fires.
Third, when case gaging your rifle ammunition after loading, as it’s sitting there inside the gage primer-side up, take that opportunity to run your finger over the primer, make sure it’s fully-seated, ideally with the primer cup actually slightly below the base of the cartridge. High primers, where the primer sits with its top actually above the cartridge base because it wasn’t fully-seated during the reloading process, are a major source of slam-fires as the bolt hits the high primer, therefore mashing priming compound between the primer’s cup and anvil.
Reloading rifle cartridges that are perfectly safe to fire in bolt action and semiauto military-style rifles is easily doable, as long as we follow a few simple rules: Do not use pistol primers, use Magnum rifle primers, check for high primers, and you should never have a problem.
