Parallax - In telescopic sights, the condition that exists when the reticle (crosshairs) does not lie exactly on the image plane. Excessive parallax makes the shooter's eye position very critical if repeatable accuracy is to be obtained. Most lower power scope have the parallax pre-set at 150 yards; high-magnification scopes (10X and up) commonly have an adjustable objective to correct for various distances.


Patched or Paper-patched Bullet - A bullet with a wrapped paper "patch" commonly used in older black powder cartridges. Derived from the even older cloth patch used to wrap a muzzle-loader ball. The patch helped seal the powder gases and reduced bore leading, and was an evolutionary step towards today's metal bullet jackets.


Pattern - The way a shotgun places its shot load. Generally measured as the percentage of pellets that strike in a 30" circle at 40 yards. Has been jokingly used to describe a very poor rifle group.


Pierced Primer - A primer that has been punctured; caused by a defective firing pin, a weak firing pin spring or excessive clearance between the firing pin and breech.


Plinking - Informal target practice commonly at informal targets. Shooting for fun where no one keeps score.


Point of Aim - That point on which a gun's sights are aligned; the intended spot where the shooter wants his bullet to strike.


Port Pressure - In a gas-operated firearm, the pressure measured at the gas port leading to the piston assembly.


Powder - The propellant material used in most gun systems. Divided into two basic types: smokeless powder and black powder. It is produced in a wide variety of types, forms and brand names intended for specific applications. It varies chiefly according to burning speed. The fast-burning types are used for light bullets in short barrels at low velocities: slower-burning powders are used in longer barrels and in greater quantities to drive the bullet at higher velocities. Most powder contains a major percentage of nitrocellulose, with small traces of other compounds intended to control burning rate or prevent deterioration; such powder is called single-base; smokeless powders containing a percentage of nitroglycerine are called double-base. Powders containing substantial amounts of other organic nitrates are called multi-base. Further identified by shape of individual kernels or granules. See ball powder, flake powder and extruded tubular powder. Black powder is a mechanical mixture of sulfur, charcoal, and saltpeter. It is now used primarily in muzzle-loading guns.


Powder Bridging - A condition that may occur in the drop tube of a powder measure. The powder kernels interlock and wedge together to block free passage. A "log jam" of powder. Most common with long, cylindrical powder kernels. Can be compared to an arch construction in a bridge.


Powder Measure - A mechanical device to meter powder charges by volume. Used to speed the charging process. Must be set using a reloading scale.


Powder Scale - A sensitive measuring device used to accurately weigh small charges of powder. Designed expressly for cartridge reloading, it is usually graduated to permit weighing to units as small as 1/10th grain.


Powder Trickler - A mechanical accessory that dribbles a few powder granules at a time, used with a powder scale.


Pressure - The force exerted by a burning charge of powder in the chamber of a gun. Expressed normally as the peak pressure in pounds per square inch (psi) or copper units of pressure (cup) depending on the test equipment.


Pressure Gun - Device for measuring chamber pressure generated by a cartridge. Usually of the "crusher" type or of the electronic "transducer" type. Both types were used in the development of the Speer Reloading Manual.


Pressure-Velocity Ratio - Numerical comparison of velocity to pressure in a specific cartridge with given bullet and powder.


Primer - Also called "cap", deriving from the percussion caps used with some muzzle-loading arms. In a centerfire cartridge, the small metal cup contains a detonating mixture which is used to ignite the propellant powder. The primer is seated in the primer pocket in the base of the cartridge case. The standard American type of primer, the "Boxer," also contains an anvil. Electrically fired primers are used in some military weapons and in some experimental European sporting arms. In a rimfire cartridge the priming mixture is contained within the rim of the case. See anvil, Berdan, and battery cup.


Primer Flipper - A two-piece metal or plastic tray for orienting and turning primers. Facilitates loading of primer tubes in semi-automated equipment.


Primer Indent - Depression made in primer by firing pin. Also called the firing pin impression.


Primer Leak - The venting of high-pressure gas between primer and primer pocket wall. Usually indicates a damaged primer pocket, and can erode the breech face of a firearm.


Primer Pocket - The cavity in the base of a centerfire cartridge case made to receive and support the primer.


Primer Pocket Reaming or Swaging - Two methods for removing the primer pocket crimp from military cases.


Primer Punch - A tool part which inserts the primer.


Priming Tool - A specialized tool which does only the priming operation. Usually used in reference to off-press priming.


Progressive Burning - Characteristic of a powder which burns at a predictable rate, producing a gradual pressure build-up to safely accelerate the bullet.


Progressive Reloading Press - A reloading press designed to perform different operations on multiple cases with each pull of the press handle. Cases progress through the device via an indexing system, stopping at each tool station for processing. Used to increase the output of cartridges per hour of reloading. Intended for advanced, experienced reloaders needing volume production; not recommended for the novice. Contrast to single-stage reloading press.


Projectile - A bullet or any other object projected by force and continuing in motion by its own inertia. Note: A bullet is not a projectile until it is in motion.


Proof Cartridge - A special cartridge used to test a new or repaired firearm for strength and safety. Usually about 25% higher pressure than normal maximum pressure. Not commercially available.


Propellant - The technically correct term for ballistic chemical used to propel a projectile. See powder.


Protruding Primer - A primer which partially backs out of the primer pocket on firing. Usually an indication of low firing pressure.


Pyrodex® - A recently developed black powder replacement designed primarily for use in percussion muzzle loading arms, black powder cartridges and muzzle loading cannon. Manufactured and distributed exclusively by Hodgdon Powder Company.