You keep mentioning the 9.5, have you or anyone made one from the 338 lapua case?
That was the whole idea behind the ".375 Twister" aka "9.5x69mm Tornado" aka .375 XLR (9.52x70mm)
now listed by CIP.
Started as a .338 Lapua Magnum case necked up and fire-formed to sharpen the shoulder.
Dreaded donut at shoulder-Neck 1 inside reamed as described above.
The Lapua-made .338 LM cases are tops, apparently made to tolerate 4700 bar/+68,000 psi,
as new they come with the nice annealing visible, not polished off.
.308-.458/.338 Lapua Magnum cartridges are all easily made from .338 Lapua Magnum cases.
No donut problems with .458 and larger.
For .500 and .510 calibers the case is blown out at shoulder (improved) for greater shoulder diameter.
At .500-caliber, about 1 in 100 cases will split the neck with fire-forming, even with the best-quality Lapua make.
The .500 and .510-calibers are therefore made from basic cylindrical .338 Lapua Magnum necked down.
Anneal after necking down.
The .416 Rigby basic cylindrical can also be used.
Specs may be .002" bigger on the Rigby rim and head, but that is a maximum spec and as made,
the .338 LM and .416 Rigby are about identical at rim and head diameters.
This new .375 XLR brass is specified for +68,000 psi by CIP, same as the .338 LM per CIP.
My .375 Twister could be happily made from .416 Rigby brass not meant for +68,000 psi.
You may find once-fired milsurp .338 LM brass of various makes for sale, cheap.
I believe that stuff is not annealed so the neck and shoulder are left hard in military ammo,
better function for slamming home one-time in battle (especially in a full-auto!),
meant for one firing only and discarded.
A duffer like me could use that by annealing before necking up.
Check case capacities. Milsurp might be thicker brass, less case capacity.
Sure would be nice if the .375 XLR catches on and becomes available commercially.
I won't hold my breath until then.