Get an 18" rifle gas. 6.5 CM is a very gassy round and the extended gas system is noticeable. I ONLY went with a 16" because I am waiting on a can in jail and wanted a shorter OAL.
To all the dudes who think a short 6.5 CM isn't worth it compared to a 308.... YOU are behind the curve. Sorry, but I dont spend a ton of time reading and extrapolating internet posts to figure out what gun is best. I go out and shoot. I have noticed a trend in large frame gas gun shooters all migrating away from 308. I even had a 308 for a little bit. So glad I shoot a 6.5 now. Sorry you just dont 'get it'.