What happens? that imaginary cartridge is so overbored that becomes literally a boomb wiht a little valve. It is BS in other words.
The thing is one doesn't need to do something very special to achieve amazing speeds.
Many companies and individuals have researched hyper speed cartriges and always ran in the same problems.
For rifles the main limitation starts with the bullets themselves because as one passes Mach 4 bullets start to disintegrate so they
must be built of different materials and design that might defeat the original purpose.
So this is great for military / penetration but then no bullet will be able to compete in range with something self propelled with
explosive attached to it.
Weatherby actually did a lot of testing for himself and for the Army. He necked the 378 down to 30 cal and use something like a 60gr
solid or something like that to hit 5k fps.
In reality it is not that complex and you can take your 308, buy some sabots for 22 cal, buy a few 45gr bullets (something not too thin
of a jacket ) and then let it rip with some faster powers. w/o any special equipment you will be hitting the 4500fps soon although accuracy
will not be that great.
That is why the make varmint cartriges and rifles. longer 27" barrels and something like the 204 ruger that will put a little pill to 4.5k fps.
Perhaps the fastest will be the 17 remington but in terms of popularity will be the 220 swift that perhaps at some point had the largest
assortment of fast rounds.
But a lot 22 calibers will do including the 223 WSM and a few others.
By increasing the case capacity or reducing the bore and weight one doesn't get faster w/o substantially increasing the lenght of the barrel
to the point that is not feasible in a rifle anymore or to require powders that are not available for anyone commercially.
One can complicate things A LOT to squeeze only a few fps and to find out the bullets explode due to the speed anyway.
Same reason military planes cannot go over mach 3 or 4 w/o huge problems and melting in flight and thefore they have no use other
than curiosity and research.
In order to hit massive speeds the answer is in electricity and magnetism that is what the navy has been doing with rail guns, sabots
and solid tungsten projectiles. Everything else around cartridges past certain speeds is a waste of time.