In regard to your listing of H2O water capacity, the 338 Lapua (which I assume is a std 338 Lapua??) is way off. A std 338 Lapua is very close to the 338 RUM at 113gr of H2O. I shoot a 338 Yogi (Dave Tooleys version of a 338 Lapua Improved) and I get 120.6gr of H2O on that so the parent case can't be bigger. I've measured the 338 RUM, std 338 Lapua, my 338 Yogi, and 338/378 cases personally and I agree with your numbers for these cases except of the 338 Lapua. It's almost neck in neck to the RUM case right around the 113+gr H2O capacity area.
FYI: Lapua brand 338 brass has about 1gr more water capacity over Norma brand 338 Lapua brass.
That would explain a few things. [img]images/icons/smile.gif[/img] Most notably, the lack of extra velocity out of the Lapua version as compared to the RUM.
My data came from a relatively new reloading software program. To be fair, the version I have is a "beta" version, and the data may be correct in the final version. I'll e-mail the program writer and let him know.
So Steve, what would you guess a 338 RUM's capacity would be if using the full length 300 RUM case and necking down to be? More than the Lapua, but not as much as the Yogi that has the good brass? My father is considering the 338 RUM Imp. for a LH Rem 700 action I have.
No problem. Good luck with your program and whatever caliber you decide to build.
How's it going? Sure hope your over your M1A experience ok. That was scary!! anyway without having actually measured a 300 RUM myself, I would have to say based on the known outside dimensions of that case, that would be a pretty fair assumption to say it would be right in between the 2. Should make a nice hunting rifle..... especially in a lefty version! A rifle built the "right" way.