The barrel is a tube which the bullet is projected.
The pressure of gases driving a rifle bullet
can reach about fifty thousand pounds per square inch
(3333 bar) and temperature 3500 degrees Fahrenheit
(2200 C) in the barrel.
These rise and fall during the few milliseconds
(thousandths of seconds) when the projectile is in the barrel.
So both thermal and mechanical stresses affect the barrel.
Formulas which describe gas pressure and projectile position and velocity
have been determined experimentally, for example LeDuc's method.
Pistol and shotgun rounds operate at considerably lower pressures
than rifle rounds at approximately one third.
As described above, barrels are usually rifled since the spin
imparted by the rifling stabilizes the projectile in flight,
decreasing wind deflection, and increasing aerodynamic stability.
Exceptions include shotguns which fire a collection of projectiles
with each shot, and anti-armor guns which fire light, high-velocity
sub-caliber rounds. The latter are usually fired from tanks with
smooth bore barrels (barrels with no rifling).
The latter are usually surrounded
by a sabot while in the barrel. The increased bore
diameter occupied by the sabot means pressure is
applied over a larger surface area, resulting in a higher velocity.
The sabot falls away after the projectile leaves the barrel.
The smaller projectile also has less frontal area to be affected
by aerodynamic drag and travels more efficiently to target.
Saboted slugs and bullets are sometimes used
in rifles, shotguns, and black powder small arms.
The chamber is the portion of the barrel or barrel extension
which supports the cartridge case while it is in firing position.
The trigger is the user interface to the firing assembly.
It can be activated by finger pressure, or it can be an electro-mechanical
device. In some systems the entire firing chain is electrical,
for example in most 20mm and larger cannon and a few .22 rimfire
competition rifles. In those systems the trigger is a switch
or electrical relay.
The sear is the portion of the trigger mechanism which
directly holds and releases the bolt or striker.
It interfaces directly or indirectly with the trigger.
Striker (Firing Pin)
The striker is usually a small rod or hammer which impacts the
primer of the cartridge, setting off its percussion-sensitive
charge and beginning the propellant ignition chain.
The striker assembly sometimes consists of multiple moving parts
such as a hammer which hits a transfer bar or firing pin.
Strikers are sometimes implemented as firing pins.
The more generic term striker is generally used here.
The receiver is the body or frame of the gun to which the barrel,
ammunition feeding devices, stocks or handles attach,
and in which bolt operates. The bolt often rides on
rails, rods, or recesses in the receiver.
The receiver is sometimes divided into separate assemblies
to facilitate cleaning, stoppage clearing, or other
operational issues. The receiver and operating parts
inside are also called the action.
The receiver can be made from stamped sheet metal,
cast and/or machined metal, high-technology plastics,
or combinations thereof.
One of the most successful applications of a plastic
receiver is the frame of the Glock pistol. The frame has
carbide (very hard metal) guides molded into it. The Glock's slide (bolt)
is conventional tool steel with a high-tech subsurface finish.
The bolt or breech constrains the cartridge in such a way
that high pressure gases generated upon firing are kept in
the chambered case and barrel. This allows pressure to fall to safe levels
before the action is opened to load the next round or stop firing.
The Operating Systems
section describes various ways this can happen.
Especially in designs which use rotary locking, the bolt and
bolt carrier can be separate objects. The bolt engages locking
lugs in the receiver or chamber and the bolt carrier
holds the bolt. Different methods of accomplishing
this are described in the next
The bolt carrier is usually larger and more massive than the bolt.
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