first, its an interesting experiment what can be done.

think the short case and the big bullet is no coincidense because with a long case( big powder load) and a small bullet the bullet maybe will not take the heavy rotation in the end. maybe a full metal jacket will not take the spin but disintegrad in the barrel under the extreme force so a solid bullet is a must.

this are trials what happen if you leave the "normal" like a 308 Win. the new US army 6,8 mm cartridge is a similar nonsense in my eyes. get more speed form a short barrel?
yes, can be done but only with higher pressure bringing more wear and tear on the gun.

a cartridge like the 308 Win is the common sense and it will be because physics dont change