ClarkM,
First, are you calculating the surface area, cross sectional area, or what of the barrel in square millimeters?
Second, pressure is what causes throat erosion, and pressure does not correlate with (only) powder weight. 48 grains of powder with a 30-06 will have a lot higher pressure if used with a 173 grain bullet than with a 125 grain bullet, even if different types of powder are used. I think this is a "hole" in your theory.
Also, modern high power shooters often use the .223 Remington (more powder than a .222) and 6000 rounds of barrel life is not unexpected.
Indy
