What 600 yard cartridge to rebarrel a 6ppc boltface to?

[Sniper338]

I run compressed loades in Grendel cases. I find it likes compressed more too. And very long brass life..

 

[Will Henry]

First choice would be a 6PPC in a 7.5 or 8 twist barrel. 2750 with 105's will do fine at 600. WH

 

[Ksmirk]

I've just started playing with a 243LBC and fella's I'm rather impressed! little round has a bunch more punch than I thought it would. Was fire forming some brass the other day on the silhouette range and 500 meter plates were as easy to hit as with my 243 Win and a light charge of 8208 and 95 Berger's was running 2770fps, I've not gotten real serious with load work but loads just thrown together were printing rather promising groups @100. I think that little round would be a good choice, good enough I put in an order for an AR barrel as well

 

[JimmyBarryNC]

The new federal 224 valkery is a round that takes the AR15 out to 1300 yard range it is super sonic past 1000 yards very impressive I am ordering a upper 24 inch barrel and bolt for use in my 5.56 can’t wait to take it to the range

 

[JEFFPPC]

I have a full custom Nesika 6ppc short range benchrest rifle that I love, but don't use that much as I currently enjoy LR shooting more. I am considering having another barrel spun up, but am a bit limited by that bolt face. I happen to have AR uppers in 6mmAR and 6.5grendel so i'm all set up with dies and brass for those two cartridges so those two would be an easy conversion.
At most I would use it in an informal 600 yard benchrest match, but mostly just trying to shoot small groups at 600 yards.
Would you choose 6mmAR or 6.5grendel for this purpose? Or is there another cartridge that would be worth buying dies and brass for?

Any advice on bullet weights to choose/advice based on experience with this sort of thing?

Thanks in advance.

Why not have you bolt face opened up, you can still use your PPC barrel on it and then have a whole bevy of more appropriate cartridges to choose from.