I have both a GAP-10 and a JP LRP-07 in 6.5 CM.
The GAP-10 had some major teething issues in regards to cratered and piercing primers with both factory 140gr ammo and my own loads which were loaded down to 39.0gr just to try and improve the primer condition; it went back to GAP and they narrowed it down to the larger .080 DPMS firing pin causing issues with the pressure curve in 6.5CM. Their fix was to change the barrel extension and swap to an Armalite bolt that uses a smaller .068" firing pin. (Should have the rifle back late this week but I trust they fixed it.)
I just received the JP LRP-07 last week; being it uses the .080" DPMS firing pin I was worried about primer condition given my experiences with the GAP10. It's been to the range once so far and I put about 100 rounds down the tube of both my own loads and factory 140gr ammo and so far I haven't had a single issue with it regarding primers. Then again, the JP is a 22" barrel and my GAP10 is a 24" so maybe the extra pressure & dwell time in the GAP10 with the longer barrel was exacerbating the primer issues.
Regardless, the Creedmoor case design in both 6mm and 6.5mm is great in a gas gun where you're limited to 2.800 OAL by the magazines. They load to mag length easily and feed nicely.
You can read about my 6.5 GAP10 issues/experiences here including a bunch of input from George @ GAP:
http://www.snipershide.com/shooting...vy-swipes-2-pierced-primers-factory-ammo.html
Regarding bullet selection, I'm using the 140 Amax right now but George mentions his favorite 6.5 in gas guns are the Berger 130's.