Shot a few boxes of 20g shells using some Gauge Mate chamber inserts in a 16g LC Smith. Summer weather. Not a hint of a bloop. Shells included AA and STS - they are easier to remove from the Gauge Mates. Federal shells worked OK, too, but are harder to push out of these inserts. Shot a few 16g shells, too, and could not feel or hear any difference between them and the 20g shells in the inserts. Did no pattern testing.
Tempting to think of overbore as similar to using a larger gauge. The wads of the larger gauge would contain the shot within their petals a bit better than the wad of a small gauge going through an overbore. The shot column would start out shorter in the bigger gauge for a given load. Overbore could allow use of a bit lower pressure to get the same speed as the gas seal area would be a bit larger. The bigger gauge would be able operate lower pressure and equal the speed of smaller gauge for the same load.
