But somebody already commented that the big jump in velocities with improved rounds is due to more pressure. This is true, since most handloaders using improved rounds work up loads using "pressure signs." Since most factory cartridges are limited to 60-65,000 PSI, adding another 5,000-10,000 PSI results in considerably higher velocity.
John, I was specifically referring to cartridges such as the .257 Roberts and .280 Rem that have reduced pressure standards due to chambering in non-bolt action rifles. Just loading these rounds to 65K PSI in a modern bolt action rifle provides a significant jump over SAMMI. The extra powder capacity is just icing on the cake.
