The 30 WSSM holds 56.5 grains of water to the top of the neck, the 308 holds 56 grains. The 30 WSSM will reach 2700 fps with the 168 AMax. Net gain zero over the 308. The shorter powder column is a theoretical advantage but on paper it doesn't justify the effort.
The 300 WSM
holds 79 grains, however, to shoot VLD bullets in a short action requires really deep seating the bullet.
My latest approach splits the difference between the WSM and the WSSM case. The 300 Warthog (short, fat and deadly like the A-10) runs well from a 2.860 mag well and is 1.900 case length. It generates 2900 fps with the 168 AMax with a seating depth of .290. All velocities are from 22" 1:8 twist barrels. The fast twist is needed for the subsonic 240 grain loads.
This cartridge will perform admirably in the suppressed subsonic mode. Unlike the other dedicated subsonic designs, however, the 300 Warthog can reach 2550 fps with the 240 MK and still be supersonic at 1400 yards.
The only downside I have found is the series of form dies required for the brass. So far I have the 6.5mm, 300, 338 and 458 versions and will probably add a 408 soon. I use the Savage Accutrigger WSM action and love the results.
So much for the short answer on water capacity. Sorry for the essay.