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Small Caliber

In small caliber systems (<19mm), the projectile reference diameter is nominally an interference fit with the barrel lands, so the dispersion in these systems is typically quite small in comparison to medium and large caliber systems. For these systems, manufacturing defects and asymmetric engraving due to excessive jump to the rifling are typically the largest sources of short range dispersion.

The following picture gives an idea of an extreme case of asymmetric engraving for a small caliber projectile. Shown at the top of the image are measurements of the length of the land engraved on the projectile body. For this case, we'd take the difference between the maximum engraved length (0.684 inches) and the minimum length (0.675 inches), divide by 2, and use that value as the “in-bore” clearance.

Most small caliber systems shoot dispersion that corresponds to approximately 0.005mm (0.0002”) clearance, with a similar sized center of gravity (CG) offset. The bourrelet length for small caliber projectiles can be more accurately determined by using a “land surrogate”; a barrel component which is the diameter of the barrel lands and extends aft and forward to allow the user to determine the intersection point between the bullet and the lands. The aft and forward bourrelet contact points with the barrel lands are shown here:

To determine the exact bourrelet locations, the user simply positions the cursor at the intersection of the projectile body and the land surrogate, and clicks and holds the left mouse button. The X and Y coordinates of the cursor are shown in the upper left-hand corner of the image screen. The user should make note of the X position of the aft bourrelet and subtract that value from the forward bourrelet X location to obtain an accurate estimate of the bourrelet length for small caliber bullets.

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