Fired brass for conversion to 6mm Grendel.

[StoneHendge]

I used new Starline 6.8 SPC brass and necked them to 6mm with a .285 Wilson bushing and then a .267 Wilson bushing and all went smoothly.
This was for my 6x6.8. Going from 6.5mm to 6mm should be easier. I don't think you will have any trouble. Just use a good coating of lube.
Bringing out the big gun, eh?

Edit: You could adjust your die a little at a time and try to fit it in the chamber and close the bolt with the upper off the lower and with a charging handle installed to get the best fit. Both thumbs on rear of receiver and forefingers on each side of charging handle will eject it if it gets a bit tight.

Do you know how thick the Starline 6.8 necks were before sizing down? I keep going back and forth on what I want to do.