Air Pressure and the Shotshell Pattern

By Randy Wakeman

sphere and shock wave

Few stop to consider what makes a pattern a pattern in the first place. Empty barrels are not strictly empty, for they are filled with air. It is atmospheric pressure pushing against spheres that form a pattern. In outer space, no choke would be necessary, for you would have patterns that have no reason to spread.


The leading edge of the pattern gets the brunt of air pressure. A smaller shot column diameter has less frontal area to work against, the reason chokes work and smaller bores require a bit less constriction to get the same pattern diameter.


Patterns do spread, of course, but it a combination of factors. Smaller, lighter pellets are blown away from the shot column easier than heavy pellets. Double-aught buckshot loads spread comparatively little and are least affected by choke constriction.


Non-spherical and deformed pellets have more surface exposed to atmosphere, so they blow away from the shot column quickly. The Winchester “Blindside” loads are a reinvention of the spreader loads as a result of non-spherical shot.


Faster loads spread faster than 1200 fps loads for the reason of increased air pressure again. However, shotguns shoot tighter patterns at higher elevations for the same reason: air pressure again, only this time less of it.


An important factor is how well the shot cloud leaves the wad. A.C. Jones documented particularly poor Hevi-Shot loads, where the shot became embedded into the wad, stayed with the wad and therefore unavailable to become part of the pattern. Extended parallel choke tubes allow plastic wads to open from their angled condition in the tapered section of the choke, enabling a cleaner release of the shot from the wad. In addition, there is a bit of extra friction in an extended parallel section that also promotes clean wad / shot separation, as discussed by Don Zutz in conjunction with one-piece plastic wads.


Air pressure is the primary reason for the benefits of spherical shot, shot of the same diameter, shot of the same mass and moderate shot launch velocity. By extension, things like buffered loads that reduce shot deformation, retaining sphericity, are also clearly beneficial.

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Copyright 2013 by Randy Wakeman. All rights reserved.