interesting question! i have a 7mm-08 (.308 necked to 7mm) and am awaiting an answer from Sierra on just that question. How is it that using the same case as the 308 and then necking it down to 7mm do we get some major differances in velocity with bullets of the same weight? My guess is that it must have something to do with bearing surface of the bullet in the bore of the barrel. If anyone knows drop me a line.