Tests in pressure laboratories do indicate that 30-degree shoulders result in more consistent muzzle velocities than shoulder angles much smaller or larger. These have also been confirmed on targets in various ways. Probably the most popular short-range benchrest cartridge before the .222 Remington appeared was the .219 Wasp, a shortened and "improved" .219 Zipper case with a 30-degree shoulder. The .222's shoulder is 23 degrees, but it was blown out of the water by the .22/6mm PPC's--which have 30-degree shoulders, arrived at by considerable accuracy experimentation by Lou Palmisano and Ferris Pindell.
The list of "accuracy" cartridges with 30-degree shoulders is pretty extensive--.22 and 6mm BR Remington, 6XC, 6x47 and 6.5x47 Lapua, 6.5 Grendel--but the angle doesn't have to be exactly 30. The .22-250, for instance, has a 28-degree shoulder, as do the .223 and .243 WSSM's. The .25 WSSM has a 30-degree shoulder, for whatever reason, and while the SAUM rounds never did well in their original form, necking down to 6.5mm resulted in another very accurate cartridge.
