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Matabele
When considering conversion of a 28 vs. 20 gauge shotgun to .303 British, it isn't simply a matter of choosing frame size most suitable for overall rifle finished weight. One has to take into careful consideration the following: Shotgun frames tend to be less robust, smaller in general, than double rifle frames of same basic outward size, to hold up to increased stresses associated with rifle calibers. Many Italian makers of shotguns and double rifles indicate in their catalogs that they use same frame size for double rifles as they do for shotguns of a specific gauge, BUT REINFORCED IN AREAS NEEDING SUCH, TO HOLD UP TO ADDED STRESSES OF DOUBLE RIFLE CALIBERS. The word "reinforced" needs to be emphasized, due to its importance--they are reinforced in specific areas where needed. More importantly, the type steel that was used in making an action is paramount in its strength to hold up to the stresses involved, all steels are not equal in this regard. Most shotgun frames, and early double rifle frames too, were made from a mild steel, a low carbon steel, color case hardened. The color case hardening provided a very hard, but extremely thin, outer shell, and color, over the action, to resist surface scratching, surface wear from friction, etc. but provided little or no added tensile strength to that mild steel frame. This color case hardened mild steel retained a core that was left in its original soft state that was springy to a certain degree, so it would flex ever so slightly under firing recoil, but would immediately spring back to its original dimensions, as long as firing stresses were kept within range rifle was originally designed for. In more recent years, almost all double rifle frames, and some shotgun frames too, have been made from more modern high tensile strength alloy steels of vastly superior tensile strengths, compared to the earlier mild steels that were used. Even though such modern high tensile strength steels may be used by most makers, they are still reinforced, where needed, for even more insurance that they will hold up well over time, to the rifle stresses involved. All, or nearly all, modern double rifle makers today, are making their double rifle frames from a high tensile strength steel. Thus, a shotgun frame made from one of these high tensile strength alloy steels, generally is much better for conversion, rather than using one made from the more common mild steel. Two examples of shotguns that used high tensile strength alloy steels are the Browning BSS, and CZ. Conversions of CZ shotguns to double rifles, were used by one outstanding maker who posted details of his conversions on this site. In general, I would judge the 28 gauge shotgun frame to be too small for .303 conversion.