The center of form of a bullet is generally defined as the point at which all of the axial forces could mathematically be concentrated from their actual vectors. The center of gravity of the bullet is the point where the bullet could be balanced against gravity. Although the actual center of gravity and center of form are located inside the bullet, we can simulate the location of the center of gravity by attempting to balance the bullet on a razor blade. The point along the side of the bullet where it most nearly will balance is close, in practical terms, to the actual center of gravity.
The center of form is harder to ascertain by external testing, because it is a measure of the forces working upon the bullet as it flies through the air. However, we can shift the center of gravity within the airframe of a given bullet by using a combination of two different densities of material in the core, each in its own separate “compartment”, so that the ratio of length of each will determine where the center of gravity lies.
For instance, linear polyethylene balls are available in various diameters that will slip easily into nearly any caliber of bullet jacket. These polymer balls will compress under swaging pressure to fill the space available, yet add only a few grains to the total bullet weight. Filling the entire bullet with two or more plastic balls creates a standard length of bullet that has only a few more grains than the jacket itself. A typical example would be a 58 grain .308 rifle bullet, which is normal length for a 168 grain bullet. Another would be a 40 grain 9mm which has the appearance of a normal 140 grain bullet.
These light bullets can be fired at extremely fast speeds with the correct charge of fast burning powder, but they generally are not accurate.
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