I have a 256Gibbs Mag built on a M/Sch by Geo Gibbs Co.

I make brass from 8x57 (cause I have it available.

I size the neck down and push the shoulder back in a shortened 6.5 Swede Mauser FL die.
I find that I must put a taper on the case or better said, narrow the case body from the shoulder an back down a slight amt. I use a 303Brit FL die to do this.
I just use an educated push of the case into the 303 die to get the case tapered down a bit.

Unless I did this, the untapered case would give a false HS feel as the wider shoulder hits the chamber edge. This results in pushing the neck back till the brass chambers and the bolt closes with a bit of resistance which is what I wanted.
But what I had was the shouder pushed too far back and the neck too long and the new case HS'ing against the side wall of the chamber.

It didn't look out of place, but when those orig few cartridges I made up w/o the tapering step were fired, the fireforming produced a cracked shoulder on a couple and the obvious difference betw the fired and unfired case told the story.

All goes well now.
I use 6.5 Swede loading data. 160gr FMJ pulled military bullets.
Excellent accuracy with my Start loading data, it's just a range toy anyway.
I probably didn't need to shorten that 6.5Swede FL die as I had orig, but it was an orphan anyway and still is used to neck size the reloads.

I have seen it said that the 6.5 Gibbs Mag was based on the 6.5 Swede Mauser cart, but the larger base dia of that round will not enter the chamber of this particular rifle.
It's a standard Mauser x57 base dia.

The orig cal of the rifle was 6.5x54M/S with it's smaller base dia (.421?). A surplus Greek WW1 rifle.
The orig Greek M/S magazine is in place and the cart slots are not altered for the slightly larger x57 Mauser round. So they sit atop and straddle the cartridge cutout slots in the mag. But it feeds all 5 just fine.
The cartridge guide ring at the front of the magazine has been reduce/thinned on the inside of it's dia to allow this. I did a bit more work on it to smooth things up. The ring is removeable with 2 small flush headed screws. IIRC when I reinstalled them, I soft soldered them back into place with the ring for extra stength as they are very small and short threaded.

The cartridges sitting a bit higher puts the bullet higher as well.
If I seated the bullets deeper and placed the curve or the bullet (ogive?) under the ring, the ring could have gone unaltered.
Lighter weight bullets would have the same effect .
The orig cal cartridges sit full down in those magazine slots and place the bullets just under that ring at the front, so they feed and rotate within the magazine without a hitch.

Now to fix the detachable 'scope that came on it so the mount doesn't place it at a Zero of about 500yrds.