For what it's worth, there have been commercial bullets made with cup bases, actually a slight dimple in the base maybe 1/3 of diameter deep. I had a batch of Norma steel jackets 6.5s years ago. I still have several hundred .318 bullets with conventional jackets with the cup base. These are repackaged bulk, but I think that I was told whan I bought them that they were made by Norma.
I have found no difference in accuracy between these and flat based bullets, but I have not shot them in anything that would show a small difference. I have no idea why they were made in that form.
If anyone wants one as a curio or to examine, email me an address and I will send you one.