The Berger 140 gr VLD is one of the better BC 6.5 bullets out there. I know I can hit around 2700 fps in my 6.5mm WSM with the 15" Lilja barrel. I would thing with an 18" barrel you may be able to sneak out another 100 fps possibly. I have heard the new Norma 270 WSM cases are hell for stout and will take alot of pressure. I have not tested this personally and to be honest, most Norma brass is very soft but there have been enough reports that there must be something to it.
Basically, take a 270 WSM case, neck it down to 6.5mm and improve the crap out of it to get as much capacity as you can in that short case and see what you can get out it. You may be close but I do not know if you will get there or not.
Richard has a 168 gr bullet in 6.5mm but the nose forming die was sent wrong, something like a 10 ogive instead of the ULD. As such, BC is much lower then it could be.
Anyway, it will be a challange in the 6.5mm bore.
You could always take the 7mm Dakota and neck it down to 6.5mm. Overbore, yes, but usible, certainly. Heck, my small 6.5mm magnum, my 6.5mm Allen Xpress is based on the slightly longer 300 Dakota case and it works just fine, the full match of the 6.5mm STW.
Will the barrel life be limited, to some degree but it is certainly practical with todays ultra slow burning ball powders. Would you see much advantage over say a WSM in the short barrel, some but not much.
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