I used to have a 6.5 international rimmed, it is a sharp shouldered 30/30 necked down. Case capicity is slightly less than the 6.5 JDJ. The barrel was a re-chambered super 14 6.5 TCU. I tried many different powders under the 120 gr bt, RL 15, Varget, VV N-550 and Hodgdon 4350. All powders were capable of 2400 fps, but H 4350 provided the best accuracy. I know 4350 would apparently be too slow of a powder, but it really seems to work well 6.5's of this case capacity.

As far as carbine performance, I've found that 50 fps/in is a good unit to estimate velocity gain/loss when going shorten than 20" in bottleneck rounds. So, a 20" tube should gain around 300 fps over a 14" pistol, hence 2700 fps with a 120 gr, 2400-2500 fps with a 140 gr. That is a far cry from the .260, but will be on par with the 6.5 PPC.