I have the HS issue all set.
After I fired the first few of the above 6.5Gibbs I made up I realized the shoulder was pushed way too far back and I was seeing my home=made case had headspaced on the side wall instead of the shoulder.

That's when I put the extra step of tapering the case with a 303Brit FL die into play.
I mentioned that in th first post.
That elliminated the false HS reading and put the shoulder itself back in the game.

Then it was easy to form 256 Gibbs Mag with a correct shoulder to HS against.
They still fireform out a tiny bit first time as the taper I put into the case body isn't exact, but minimal to allow correct HS against the new shoulder.
Then simply neck size when reloading from there.
I use 6.5 Swede dies for loading. The FL Size is shortened for this.

I couldn't find any of the new 256Gibbs cases handy to show, but they work just fine.

8mm Mauser brass makes a case that requires no thinning of the neck. The ones in the pic were made of Surplus 30-06 and I did have to thin them. These were outside thinned.
Using 8mm Mauser brass also requires that I run the formed case into the 303Brit die to taper the body down a bit.

So as far as I can see working with my own rifle..
The 256 Gibbs Magnum is made on a standard 30-06 or x57 Mauser case base dia.
Not the slightly larger Swede Mauser case,,and obviously not the small 6.5 M/S case.
At least for my rifle/chamber, the body of the case has slightly more taper to it than the donor case
may have with the correct head size to work from. The 303Brit FL die takes care of that easily.

Was all that worth it to get that cartridge?, I don't know but it shoots very well and it's a fun rifle to reload and 'play' with.
Like most every other propietary cartridge or wildcat, there's a half dozen others around that do basicly the same thing.
But life would be boring with out lots of choices..