This is a matter of ratios. Rifling, absent or present, is practically irrelevant. Throats don't have much rifling anyway.<P>The ratio of (a) the circumference of the cross-section of the bore to (b) the area of the cross-section determines the rate of erosion (as far as the bore dimensions are concerned). Stated another way, it's the ratio of (a) the surface area of the bore to (b) the volume of the bore. Since length is common to both the surface area and the volume of the bore, length cancels itself out of the ratio.<P>Call this ratio C/A for the ratio of circumference to area, or S/V for surface to volume. The C/A for a .224 barrel is larger than the C/A for a .244 barrel, which is larger than the C/A for a .257 barrel, etc. The C/A for a .375 barrel is minute compared to the C/A for a .224 barrel.<P>The larger the C/A, the faster the heat transfer from the powder gas into the steel. <BR>The larger the C/A, the faster the erosion at a given peak pressure. To compare the innate erosion rates of any two calibers of barrel, the ratio is (c/a)/(C/A) when c/a is the ratio for the smaller bore and C/A for the larger.<P>The barrel-makers' rule of thumb for rifling is that the lands occupy 25% of the circumference of the bore, grooves 75%. Grooves are so shallow � lands so low � and their widths are proportional � it's practical to ignore rifling in calculating these ratios.<BR>
