Steve,
The .223 Weatherby (like the .223 AI) did NOT get those velocities at those pressures, which is why Weatherby abandoned it.
The original pressure-testing was done with strain-gauge equipment, which can result in unreliable pressure readings, due to several factors, often lower than actual pressures. Strain-gauges can work fine, but since there wasn't any "reference" ammo there was no way to check the original load work-up. More work in other labs indicated pressures were much higher.
This happens occasionally. Chub told me years ago that the Nosler lab's results with the .257 Roberts Ackley Improved showed higher velocities with less pressure than the .25-06, which has at least 10% more powder room than the .257 AI. But neither of the two major piezo pressure labs I'm acquainted with have ever been able to find any magic velocities due to case shape. Instead they report that the only way to get more velocity at the same pressure is to use a larger-capacity cartridge.
The reason many wildcats SEEM to produce the same velocity as larger cartridges is traditional "pressure signs" normally don't appear until pressures are in the 70,000 PSI range, and often even higher. This is exactly why 7mm STW velocities reported by various shooters (including Layne Simpson) dropped around 200 fps when Remington introduced the STW commercially. Widely-used handloads developed far more pressure than SAAMI considers acceptable for ammunition to be fired in a wide variety of rifles in a wide variety of conditions.
