Re: easy open bullets for 6,5mm
Although I do not have specific information about 6.5mm bullets at subsonic speeds, I do know a fair amount about 30 cal bullets at subsonic speeds based on my work with the 300 Whisper.
After talking with technicians from nearly all bullet manufacturers, I can say with a high degree of confidence that there is not a 30 cal hunting bullet out there that will expand at subsonic speeds (1050 fps and under). I strongly suspect the same is true of 6.5mm bullets. Varmint bullets will not and neither will the green/safety bullets filled with powdered metal. I did find one small custom manufacturer that makes a 30 cal bullet that expands at subsonic speeds - but they are $1 apiece and he doesn't make any in 6.5!
Most 300 Whisper owners rely on tumbling for initial shock value. This usually means heavy-for-caliber bullets like the 240 gr. SMK. They barely stabilize with a 1 in 8 twist barrel at slower speeds and then upset easily when they come in contact with media. Other owners load a snub or flat nose bullet (like those made for the tubular-fed 30-30 Winchester) to minimize the "penciling" affect of spitzer point bullets. Unfortunately, I don't know any manufacturers that make a flat nose in 6.5. Finally, there are some that buy flat based bullets and then load them backwards like a wadcutter in a pistol cartridge. Again - the idea being to get the bullet tumbling shortly after contact. Some have reported that feeding backwards bullets becomes unreliable in some AR platforms.
One final thing to keep in mind when shooting subsonic is load density. Because it takes only a few grains to get a bullet up to subsonic speed, you will find most cases in 6.5 have too much excess case capacity and therefore, results will be erratic. Go with the smallest case you can find. Also, go with a fast-burning powder for more uniform results. For example: the 300 Whisper is a necked-up .221 fireball case (slightly smaller than the .223 case). I load 8 grains of Hodgdon Lil' Gun with a 208 gr. A-Max on top and easily get to 1050 fps.
Good luck to ya!