I really don't dislike the .338 Win Mag - it's probably the best of the 2.5" mags. The 8.5x63 isn't there to compete with it, like I noted, those are it's ballistics at probably the same pressure level as the factory .338 is supposed to be delivering.
I've a friend who is a staunch .338 shooter, but he prefers to hunt with is P-14 8x68 S Custom rifle - always has, yet he's also always had a .338 or 2 since I've known him - just over 30 years now.

He's also owned my first .375/06IMP after I re-chambered it for the 9.3x68 case, using his 6.5 x 68 reamer along with a neck throater in .375. That one was quite special - running 2,950fps with 270gr. He's also owned a couple other wildcats I dreamed up - however - point is, he always owns at least one 8x68S and at least one .338 WinMag. Keith is a highly practised reloader & if anything might even shoot more than I do. To do a lot of shooting, a person of normal means MUST handload & he does - although he has a case (1,000 rnds) of 8x68S factory RWS in his lockup.

Back to the .338's 64,000psi and the brass. Not all chambers are really bad - a high percentage are though, of those belted magnums I've measured up. The worse is probably the .300 Win Mag. and the .350 Rem Mag for belt measurments, ie; headspace but this isn't restricted to RP brass.

If I was going to build a .338 Win Mag (probably not) I'd use a .338 neck throater and a .300 Win Mag reamer. The reason for the .300 reamer is to leave the belt off, just as I did for the .375/.350 Rem chamber. Turning belts off is a 10 second job on each case and has to be done only once. Headspace is on the shoulder where it should be and my standard is '0' headspace, not .002", or .006", but 0.

I've never had brass go tight and I never touch a shoulder when sizing for re-loading. I use either neck dies I've made, or short-stroke FL dies.

A 'tight-chambered .338 would do very well and the brass would last for many years. Oh yeah - about Keith - he runs just over 2,700fps with 250's in one of his .338 (24" bl) and something like 2,680fps with his remaining stock of 275gr. Speers, his favourite moose bullet in .338. He turns a cannelure where it needs to be and the bullets work like H-Mantle's or thoe TUG's only with 2/3rds of the body remaining. It's a fast barrel, but it's barely 100fps faster than the 8.5x68 - with some bullet weights or loads.

The presssure limit of around 65,000psi is established by the strength of the brass - it's elasticity point. When they stick, they are not shrinking back the normal .001" off the chamber walls, nor the .001" in length from the bolt face - maximum pressures have been exceeded. Some designs of brass have their maximum pressure level as well.

Fortunately for the 8.5x63 and the various wildcats on the '06 case, along with the 9.3x62 brass, their limit is higher than the old SAAMI's or CIP's for the original ctgs when they were designed. They are capable of withstanding any reasonable pressure and the results are quite surprising.

Today, the 9.3x62 has absolutely no flies on it - 200fps to 250fps is an realistic goal for an increase over factory ammo with 286gr. It is virtually an improved '06 case in .366 calibre with shorter neck. (yes I know it's larger in the base). 2 factory fired rounds I measured were 78gr. capacity. The 8.5x63, with a similar case capacity, is behind only what the reduction in bore size dictates.

It's an excellent round, producing performance well above it's 'size' would indicate. It is especially for those who don't want the much fatter cases of the 2 1/2" mags. with their restricted mag. capacity, and belts, yet desire similar balistics.