I am familar with the 12x75R shotgun cartridge concept.
understand that someone grab this brass you mentioned out of the factory like the primed 22 lr cases you find sometimes.

you would be to short with 9,3x53R brass open up to .458 but you will allready know this. two weeks ago I was making 10,75x52R brass from 8x56R and 53mm was the maximum left over from the 56mm it starts. I am dare to say you would not get more than 51mm from a x53R case.