Dictionary Definition
obturate v : block passage through; "obstruct the
path" [syn: obstruct,
impede, occlude, jam, block, close up] [ant:
free]
Extensive Definition
Obturate means to block or obstruct. In
conventional English usage, obturation refers specifically to the
condition of being obstructed or occluded or the action of
blocking, stopping, or filling something up. The mechanism by which
an undersized soft metal projectile enlarges to fill the barrel is,
for hollow-base bullets, due to expansion from gas pressure within
the base cavity and, for solid-base bullets, upsetting - the
combined shortening and thickening that occurs when a malleable
metal object (e.g. a rivet) is struck forcibly at one
end.
Obturation in firearms ammunition
With reference to firearms and air guns, obturation is the result of a bullet or pellet expanding or upsetting to fit the bore, or, in the case of a firearm, of a brass case expanding to seal against the chamber at the moment of firing. In the first case, this both seals the bullet in the bore, and causes the bullet to engage the barrel's rifling. In the second case, it seals the case in the chamber and prevents backward travel of gases against the bolt. The thin brass case easily seals the chamber, even in low pressure rounds like the .22 CB, but expanding or upsetting the bullet sufficiently for effective obturation requires sufficient pressure to deform the bullet material. The formula used to calculate the pressure required for solid base bullets is:- Bullet's BHN x 1422 = Pounds per square inch
The constant 1422 is an empirically determined
factor used throughout the ammunition and reloading industry; it is
specifically tailored for calculating the pressure required for
expansions that occur in internal ballistics and should not be used
for other purposes. Below is a chart containing various bullet
alloys, the BHN, and the PSI required to expand a bullet to the
bore:
Pure lead is very soft, and can be expanded or
upset by most firearm cartridges, but the pressures required are
higher than those encountered in most airguns. To allow obturation
in airguns, pellets use soft alloys in conjunction with a thin,
concave base designed to expand more easily. Some firearms
ammunition, such as Foster slugs
and hollow base wadcutter bullets, also use a
hollow base to allow the bullet to expand and conform to a barrel's
irregularities, even as the chamber pressure drops as the bullet
travels down the barrel (see internal
ballistics). For example, it is not uncommon for revolver barrels to have a
slight constriction at the breech end where they thread into the
revolver's frame; a hollow base bullet will expand to fill the
larger diameter of the barrel after passing through the
constriction.
To prevent excessive deformation in high pressure
rifle and magnum pistol cartridges, lead bullets
are often covered in copper or another harder alloy. These bullets
are generally designed to swage to fit upon firing.