This isn't a 6mm ARC vs 6.5Grendel post (though it'll probably become one) - but rather a reloaders technical post on brass conversion conversation.  As everyone knows, 6.5Grendel and 6mm ARC brass are fathered from 7.62x39 brass, and can even be converted from one to the other within that family by the reloader.

Here's where it gets interesting, for whatever reason, 6mm ARC runs a slightly shorter headspace than 6.5 Grendel.  What that means, is that 6.5 Grendel brass is one press stroke away from become 6mm ARC brass.  But going the other way, is a little more dicey, since the resultant 6.5 Grendel brass will will now have too short of headspace if it's coming from 6mm ARC brass.  

In my own case, I like 6.5 Grendel. I'm not going to trash-talk 6mm ARC, it's a very cool round, but my own interest's still align with 6.5 Grendel as a little more broad general purpose round, and a game-hunting round.  And one that's not so easy to find brass for.  So, as a reloader and occasionally brass scrounger, I'm starting to see a decent amount of 6mm ARC brass on the ground (not a lot, but some; probably equal to if not moreso than Grendel brass in fact).  So, I've been grabbing it, to convert back.  But now my headspace is no good.  I probably can fire it, but broken shells are likely, and this is just not the way to go.  

Now I'm curious.  As a reloader, what are my options....

Here's my thought, and I figured I'd run it by the forum:  I'm thinking about seeing if I can force it first up into a 6.8 SPC sizer die (Which I have) maybe half-way (not all the way), to swage down the top half a bit, which I suspect will swage up the shoulder forward a bit - I think.  Then run it through Grendel dies.  Which I think will result in a false- but correct location partial headspace, that will fit the 6.5 Grendel chamber correctly.  I suspect it will fit well enough, to swell the casing shoulder to 6.5 Grendel dimensions, without leaping the whole case forward - and thus give me non-breaking 6.5 Grendel sized brass upon fireforming - without a grossly stretched mid-body ripe for a case-head-separation.  

That's kind of my theory, but I haven't tried it yet.  I don't really have that much of the brass and I'm not actually poor - but more of a scientific question of: would that work?  thoughts?

You can upsize to something larger than 6.5 in the neck, and then use the Grendel die to resize back down to 6.5.  the result will be a slight donut or oversize at the shoulder neck junction, to provide headspace.  This is all theoretically doable.

The issues are these.  You're working the shit out of those necks.  Expect splitting, neck cracks, etc.  This can be minimized by annealing .  But several anneal cycles on top of the upsize and then downsize is a lot of work.  Your resulting brass is STILL not finished, since you need to fireform to blow out the shoulder and fill out the case

It can be done.  Is it worth it?  Nope.  Not to me.  Not when commercial 6.5 G can be purchased easily