Forming cases

[Old Bob]

I have not had any luck reforming 7.62x39 cases,  all I have tried have been once fired (probably AK) and running them through a 6.5 Grendel die and then a 6mm ARC die they wind up with a "belt" neat the base of the case and will not chamber.  Reforming once fired 6.5 Grendel brass works well with no such issues. Recommendations? 
Than you

That's the problem with trying to use once-fired 7.62x39 brass. You get can that belt near the head & also a "hump" at the shoulder. If the brass you tried to resize were range pick-ups, no telling how large the chamber they came out of was.

I haven't resized any 7.62x39 for 6mm ARC but did resize some for 6.5 Grendel. Here's my experience with that endeavor...

I didn't get the belt with Winchester, PMC or Remington 7.62x39 brass I fired in my SKS. I did get the hump though. Fire-forming will get rid of the hump. However, I did get the belt with Lapua brass. Pre-measuring the brass before sizing I found the Winchester, PMC & Remington brass all had .441" head diameters while the Lapua diameter was .445".

My recommendation... stick with Grendel brass until you can get real 6mm ARC brass.