Re: .300 Lapua Mag.
According to RCBS.load software, the two cases, loaded to 3.6" COL with a 1.4" long bullet, the RUM case has about 97g of water below; the Lapua has about 103g.
The way I drew the case is by simply necking down, then bringing the top of the shoulder back to the 30* point, leaving the body the same length. Of course, you could move the shoulder forward as well. And you can gain 3.5g simply by taking that "excess " taper out. By doing that, then moving the shoulder to the point of the RUM, you get a little over 116g [img]images/icons/shocked.gif[/img] !
So, it depends on what you want to do.
But to back up a bit, I would look at your rifle, how long a bbl you're thinking of, which bullet you want to shoot, and check out the RUM's ballistics. Their's faster out there, but it is a screamer as-is. And much cheaper than custom reamers, dies, Lapua Mag cases, etc.