Ignition - The process whereby a cartridge is fired: the firing pin strikes the primer, igniting the priming compound, the flame of which passes through the flash hole to ignite the propellant in the cartridge case.
Ignition Time - The interval from the impact on the primer until the pressure rises enough to start the bullet from its seat.
Incipient Rupture - The partial separation of the cartridge case in a circumferential direction above the head.
Indexing Targets - The method that a sniper team employs to identify targets within its effective field of fire.
Inverted Anvil - Anvil in the primer upside down. This condition will cause a misfire.
Inherent Accuracy - The inherent ability of a specific calibre to be more accurate than other calibres in the same class. For example, the .308 is considered to have more potential accuracy than the 30-06, because of its smaller case. The reason is that the powder for a full charge, *completely* fills the .308 case without having to compress it with the bullet. This gives a more consistent powder burn profile than the larger 30-06 case, which has room for the powder to slosh up and down inside the case, causing each shot to be a little different. This is mostly relevant at bench-rest-level situations, where you're measuring fractions of an inch in group size differences. For hunting purposes, it's not really significant.
Internal Ballistics - The science/study of ballistics which deals with all the aspects of the combustion phenomena occurring within the gun barrel, including pressure development and motion of the projectile along the bore of the firearm, i.e. all events until the moment the projectile exits the muzzle. Also see Ballistics, External ballistics, Terminal ballistics.
Iron Sights - A term that is broadly used to describe metallic sighting instruments, covering common open sights as well as aperture-type receiver sights (as opposed to scope sights).
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