In theory it's just one expander ball (or swage) away from being 6.5 Grendel - as it's otherwise the same case.  But wait, maybe not - as the headspace for 6mmARC looks like it's knocked back a bit, moreso than 6.5Grendel.  Meaning it's going to be short headspace, if you try running it in Grendel.  I'm curious if anyone has run it anyway?  My guess is you'll get case-head separation pretty quick - maybe even on the first firing?

I have a couple of 6.5 Grendel’s, plus a .22 Grendel/.224 Predator (formerly a wildcat, now the .22 ARC). I haven’t messed with the 6 ARC at all, but as you mentioned, there are a couple of minor differences in the 6 ARC and the 6.5 Grendel.

After you expand the 6ARC neck to 6.5mm, you would basically be fire forming the 6ARC brass to fit a 6.5 Grendel chamber IF it works?

I’ve fire formed rimmed cases with reduced powder charges to a different chamber configuration (.30-30 brass necked down to 7mm then fire formed to 7-30 Waters to move the shoulder forward), BUT I’ve never done it with a rebated rim case. IF you tried it, I would do it with a reduced powder charge the first firing. BUT I’m not suggesting you try it, maybe wait for someone else to weigh in first.

When I fire formed .30-30 cases to 7-30 Waters, I would lose 1 or 2 cases per 100 rounds to split shoulders, but again, that was a rimmed cartridge being supported at the rim, in a TC Contender.

In your scenario, I don’t know if the extractor is going to hold the 6ARC case in the bolt head, OR if the 6ARC case will go deeper into the 6.5 Grendel chamber and not fire b/c the firing pin doesn’t reach the primer?  Maybe if you load the 6.5 bullet and seat it to a long OAL to have the bullet engage the rifling to keep the case head engaged in the extractor and bolt head?  

Please proceed carefully, IF at all, my limited experience may not fit your situation. I’m sure someone else will weigh in soon.

Won't work. 6arc shoulder is too far back, you'd have waaay excess headspace.

Sell the 6arc bread and get grendel brass.

Jam the lands for first firing.

ETA: looks like 0.040-0.050" difference. Dunno if annealing will make it hunkeedory or not matter and the stretch ruin them.

Going the other way (6.5G formed to 6ARC) includes trimming it by a bunch. I forget how much, could have been 1/16" to 1/8".
Not sure how important untrimming it going 6 to 6.5 is, could be a big deal.

I don't think you will like the results. Your trim length is going to be short and you be pushing the shoulder out .020 making the trim even shorter. 6.5 Grendel to 6arc works quite well, just have to trim .040.