Hey Al,
Which one are you talking about, the 300 Ultra or the 30/338 Lapua Imp I described above, so that we are on the same page?
I did some figuring on a program of mine that will determine case capacity from measurements, and I found some interesting things checking the following cases; 338 Lapua, 338 Lapua Imp, 338/416 Rigby Imp, 338/416 Rigby "Super" Imp, 300/338 Lapua "Super" Imp, 300 Ultra and 300 Ultra "Super" Imp.
I measured the water capacity of the ones I had on hand, those were the 338 Lapua, 300 Ultra and the 338/416 Rigby Imp. I verified the capacities with the program and they did match with a very slight alteration to case thickness at the base, the rest I calculated on the program only modifying the length to bottom and top of the shoulder as well as width of the body at the shoulder. The neck was reduced on the 30/338 Lapua Imp as well.
The 300 Ultra "Super" Imp, it was the only one that showed it would not be worth it to me, an increase of ONLY 1.6grs capacity was obtained, and that was with a .260 long neck, .008 per inch taper, .018" total, and a 35 deg shoulder, something I consider just about maximizes the original case, hence the "Super".
I moved the shoulder forward .100" on the 338/416 Rigby, which has a long neck to start with, and reduced the total taper from .024" to .020", this took capacity from 138gr to 143.3gr water. This was a worthwhile improvement, I thought.
I ended up with 119.8gr capacity on the 30/338 Lapua "Super" Imp using .018" total taper and .280" long neck and a 35 degree shoulder. The same design necked up to 338 gave me 120.6gr capacity, only the neck will be about .260" long.
Case capacity in the 338 Lapua was 112.3gr (virgin cases), the 300 Ultra was 115.5 fire formed and 117.1gr improved.
Thought you all might find this a little interesting.