I wonder what would happen.

Severe premature throat erosion.

.338 Talbot

250gr @ 3700 fps = 7625 ft/lbs

Yeah it'd be really overbore. An even better idea would be to proportion the .308 to the .50 BMG. Since the BMG cartridge is basically a .30-06 blown up in proportion to it's size, they should blow up a more efficient .308 cartridge to what would be it's relevant size and sharpen the neck angle. Then it needs to be adopted by all the militaries that use HMGs and go into general distribution and surplus. Either that, or start chambering rifles in the Russian 12.7x108 cartridge, which is arguably moar efficient and mounds better ballistically than the .50 BMG. Make match or at least high quality ball rounds for it.

.408CT necked to .338 with a 40 degree shoulder (a.k.a .338 Allen Mag) drives a 300gr matchking at 3300fps for max loads in a 30" tube and i know it's recorded atleast a handfull of kills between 2000 and 2500 yards, makes me wonder what you'd want a .50/.338 for, Going for that 3 mile kill? :D .

But yeah, it would have very, very short barrel life and replacing a super match lilja or w/e ever 500 or so rounds isn't my idea of fun.

"Super Maximum Hyper Velocity" is what I'm going for. :D

If you could handle a .50 BMG with out a muzzle brake then you can shoot saboted .308s.

"Super Maximum Hyper Velocity" is what I'm going for. :D

In that case the 6.5/20mm with 2 degrees of body taper, a 40 degree shoulder and 6.5mm long neck is the only way to go. add to that a 60 or 70 inch full profile barrel, 5 or 6 mates to help carry it and you've got something that has a good B.C, burns 400+ grains of powder per trigger pull, and drops about 3" over 1000y....not sure where you will find 6.5mm projies that hold up to 10,000 fps though :D .