Then don't use brass!

Steel works as russian ammo shows but it comes with issues like not flexing, shrinking once fired, leading to cases more or less stuck in the chamber. The new 6.8 case uses a two part steel head brass cylinder front mechanically locked together to get around the high pressure steel case problem. But it's not an easy or inexpensive fix. And the new 6.8 rifle is showing some teething issues in the field now.

Caseless or telescoping may be a thing but you still have the whole how to seal the pressure vessel that brass currently does. The brass case is essentially a gasket on a piston head.

That portable power thing is the hold up on the electromagnetic propulsion guns. That tech has already shown the potential to surpass conventional powder driven velocities.

I think the main reason for not higher velocities is it gains u so little for the troubles it brings. Though there are people working on it. Just look at the calibers u mentioned, they haven't exactly caught on despite their speed.

I have a Savage 110ML that uses smokeless powder in a muzzleloader. No case. Approaching 375 H&H ballistics.

Imo it gets to the point where there just not much point in higher velocity. Can get my 17-5.56 kak up to 5k with a 15.5 GR bullet but why ? Lower brass life , lower barrel life ect.

The ELR and UELR guys are doing. Some really cool stuff but there to the point where it's basically man portable artillery.

Now on a side notw these a few guys taking the .277 furry bi metal case and rednecking them and doing some ...very interesting things swore I saw one guy running a .243 at close to 4500fps but...

It's a more or less, minus the flash hole, a sealed vessel. The flash hole has a brass cap/primer and iirc that one is an inline locking bolt for a more positive stop on the primer.

Standard rifles are unsealed with the chamber. The brass case, supported by a locked bolt, functions as a sealing gasket.

Caseless ammo would require some sort of sealed vessel and steel on steel has always been problematic at getting a complete seal. It can be done as the big 16" rifles on the battleships were essentially caseless with just powder and shell.

Other problem with the 5k+ is you probably won't see it legalized for hunting outside of out west.