IRON SIGHT

Any sight, front or rear, not containing optical magnifying elements. It may be fixed or adjustable. Also called Metallic Sight.

INERTIA FIRING PIN

A design in which the striker or hammer stops after hitting the firing-pin, requiring the firing-pin reach the primer under its own inertia.

IRON FORE-END

On firearms with a one-piece stock, an intermediate part that holds the wood against the barrel and frame and retains the latch mechanism.

INTERIOR BALLISTICS

The science of ballistics focusing on all thermochemical and physical events from primer ignition through projectile exit from the muzzle.

INSTRUMENTAL VELOCITY

The velocity of a projectile, or a shot charge, that is recorded by suitable instrumentation located a predetermined distance from the muzzle of a test barrel or a firearm.

INSIDE LUBRICATED BULLET

A bullet having lubrication in grooves on the bearing surface of the projectile. When seated, these grooves are covered by the cartridge case.

INERTIA TRIGGER

The term used for firearms having two barrels and a single trigger wherein the inertia resulting from the recoil forces of firing one barrel causes the trigger sear to switch to the unfired barrel. A second pull of the trigger can then fire the second barrel.

INERTIA OPERATED ACTION

A self-loading type firearm in which components of the locking system remains relatively stationary while the rest of the firearm recoils and thereafter unlocks the breech bolt to complete the cycle of extracting, ejecting and reloading.

IGNITION TIME

The elapsed time from moment of firing-pin contact on the primer to the point on the X (time) axis equal to the point where the pressure time curve indicates propellant burning has initiated.