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Another function of the wad/shotcup is to provide a flat, regular surface against which the expanding gases can press outward. A load of shot, even target #9, is porous, and buffered shot, which is only a load of shot whose spaces between the pellets are filled with sifted polyethylene particles, will allow gas to blow through unevenly if the shot is not backed by a wad or shotcup. The wad provides a ceiling against which the hot, expanding gas can press evenly, thrusting the shot down the barrel uniformly.

The wad also seals the gases behind the shotcup in an area of limited oxygen, an element that of course is required for combustion. Thus is the burning propellant limited in its burn rate and only by forcing the shot and wad down and out the barrel does complete combustion take place.

There are numerous shotcup designs from several internationally recognized manufacturers, but your chosen formula will note specific brands and styles: Remington SP16 or WAA 12R or CB 1034-28.

When building a specific load, use the specific components called for in the formula. Why? We have discussed the fact that there are several shell types for any gauge and that even though it might not be immediately obvious, some shells are straight-walled while others have tapered walls. A wad designed for use in a straight-walled Federal hull might not work to seal the gases properly in Winchester’s compression-formed AA hulls that have tapered walls. Winchester and a few other companies make wads with a smaller, angular base to fit these special hulls. In case a wad for straight hulls is used in a tapered hull, it will usually be a little on the large size, so you will have to make sure that your press seats it snugly. (Actually, this is not so rare in load recipes, but it is quite rare that a wad designed for a tapered hull will ever be recommended for a larger, straight-sided hull.)