Benefits of the Cavity Back system
After much testing as well as consulting with
industry professionals these are the benefits of the Cavity Back in small
cartridges in gas operated firearms. Many of these benefits also equate to
projectiles of any caliber with the Cavity Back system.
1. The cavity in the rear of the projectile allows for more room for a
larger powder charge than a projectile with a typical flat, boat-tail or
rebated boat-tail design. The cavity effectively increases the capacity of the
cartridge case by typically .5 to 1.5 grains of powder weight determined by
powder density and caliber diameter.
2. By increasing effective case capacity the cavity allows the cartridge
to “breathe” better. Resulting in a slightly cooler burn and slightly less
violent activity inside the cartridge case during propellant ignition. Also
allowing for more powder before compression.
3. The area inside the cavity actually has pressure inside it just as an
even pressure acts on the outside of the boat-tail. As the projectile exits the
muzzle the gases are forcing their way past the projectile and into the air.
Some call this “blow by blast”. It is fairly well known that these gases then
pass the projectile and envelope it during the first part of its flight out of
the barrel. It is known that a rebated boat-tail or a small conical area
in the base of the projectile or a combination of the two can cause a
projectile to be more accurate. With the cavity back the gases are forced even more
outward as the projectile exits the muzzle inhibiting the turbulence and
allowing the projectile to “go to sleep” quicker resulting in a more accurate
4. The cavity system allows solid copper projectiles of the same weight as
a lead based projectile to be loaded to the same cartridge overall length. It
also allows it to be pushed to the same or higher speeds as the lead based
projectile. This is true even though the solid copper projectile is longer in
length than the lead based projectile.
5. Due to its
particular shape, the bullet has its center of mass behind its center of
pressure. When the bullet is in flight, the center of pressure always positions
itself in the direction of flight, that is, with the tail pointed forward,
pivoting on the point of pressure. If the bullet is not stabilized, these two
forces acting together will cause the bullet to tumble. We can consider the
distance between the center of mass and the center of pressure as a lever. The
longer the “lever” is, the higher the destabilizing force it causes. Longer
bullets, and especially bullet shaped with long ogives, have longer “levers”
and thus are the most unstable. The Cavity Back shifts the center of gravity or
center of mass forward decreasing the length of the “lever”. Enhancing the
Dynamic Stability of the projectile.
6. Thorough testing has proven the Cavity Back
smoothes out the pressure curve by creating a hemispherical powder containment
chamber. As the powder burns and pushes on the chamber instead of a flat or dished
bullet base the bullet starts its movement sooner due to the increased surface
area. This lessens the pressure spike as the bullet enters the rifling.