There are thousands of guys running .45 caliber WW boolits out of the modern 45/70s at 38,000 psi with excellent success, not to mention the .454 Casull, and the Lineboughs (sp?), which are pushing cast boolits at pressures near 60,000 psi. Pressure is only part of the equation, and alloy hardness is not a blanket answer to increasing velocities. Ductility has a lot to do with it and in some cases a "softer" alloy produces better results at higher velocities. If you're looking for an education in the making and use of cast projectiles go to:
http://castboolits.gunloads.com/And just read for awhile.
We are not breaking new ground in pressure, case capacity or velocity here. We are breaking new ground in in the firearms platform, and we're in somewhat uncharted waters using a fairly big case with a small primer and hard to light ball powders. I wish I could get my hands on some CC! 450s
IMHO gas checks are definitely needed in the AR platforms, not so much due to the pressures involved, but to protect the base of the boolit as it passes the gas port. I've had pretty poor luck with all of the plain base boolits I've tried, from 200 grains through 450 grains, velocities from 1,000 fps to 2,500 fps. There is a small run of gas checked 210 grain molds being produced by NOE; a hollow point design based on a 230 grain RNFP I designed for the .45 acp some years ago. I'm looking forward to trying those out as well as some more syles in the 300 grain range.
BD