The biggest liability to 6.5CM and .260 in a semi-auto, especially if it uses a DPMS-pattern bolt, is that the .080" firing pin doesn't play nice with the primers given the typical powders & pressure curves in 6.5CM and .260. Slightly cratered primers are usually the norm and if you run a little on the warm side you might pierce primers. 140gr bullet exacerbate the issue, 130gr and 123gr are much easier on the primers but still show a little primer flow & cratering.
Armalite has had the advantage for years in this department because they use a smaller .068" firing pin and the primers come out perfect with no cratering or flow. JP stepped up and is now making a "high pressure" DPMS-pattern bolt that uses a .068" firing pin. JP is now using this bolt on all new 6.5CM & .260 builds so it's no longer a concern.
Other than that... no real issues to worry about. I have both a GAP-10 in 6.5CM and a JP LRP-07 in 6.5CM. Both are quite accurate and reliable although 130s and 123s shoot better out of them based on brass condition; the safest 140gr load requires more elevation to reach 1000 than the safest 130gr & 123gr loads and both rifles would have occasional ejection issues with 140gr loads that went away with 130gr & 123gr loads. Both my rifles really like 123gr Scenars and 130VLDs.
I prefer 6.5CM over .260 in the gas guns just for magazine length limitations. .260 works fine but 6.5CM gives you a little more room to play with seating depth.
6CM is a great round too, more velocity, flatter shooting, and even lighter recoil-- but unfortunately JP doesn't offer it. I'm happy with my 6.5CM gassers sending 123gr Scenars at 2900.
