It seems to me these shells should just be rough forged right in the steel mill while they are still hot and soft bar stock. Then the rough formed blanks can be trucked to shops with lathes that can do the final clean up to shape them to specs.
Modern steel mills are continuous casting mills, which make endless ribbons of hot steel which is sheared into sheets.
Forging is a completely different technology, and while, a hundred years ago, some but not all mills also had forging plants, they dont today.
Johnstown PA had 2 or 3 forging facilities attached to its mile long mill, but that was before WW2.
Nucor, currently the largest steel maker in the USA, has 23 steel mills. They dont do any forging.
Deep draw forging like 155mm shells is a specialized forging process, and they use specific presses which are custom ordered, not off the shelf.
There are possibly a dozen big forging shops which might be able to do this, all booked up with commercial work. And it would probably take a couple of years to tool up to do this specific job.
This has always been done in government run plants in the US- there are not a bunch of commercial forging shops set up to do this.
And the heat treating is even more specialized- most heat treating shops dont have anywhere near the capacity, much less automated capacity, to make even a 1000 a week of these today.
I think the amazing versatility of modern CNC vmcs make people think they can do anything.
A VMC is a great swiss army knife for relatively small, relatively low production manufacturing, accurate, and able to change setups quickly.
but actual manufacturing, whether its engine blocks, artillery shells, or aluminum tesla chassis, can only be affordable, and able to make gigantic quantities, with specialized machines built to do one thing very well.
There is a reason why Tesla is buying more 9000 ton giga presses,rather than having some corner welding shop weld up aluminum chassis, then do all the machining on a gigantic HBM. Its faster, cheaper, more accurate, and able to use alloys and create shapes you just cant do on off the shelf machines.
Same thing with Artillery Shells- they know how to make em, they know what works, and they know that a bunch of Haas CNC mills is not the answer.