Look in any gun mail order catalog and you can find a copy of Wildcat Cartridges (combo edition). Here there is an article reviewing the ballistics and idiosyncrasies of the 6.5/300WWH. However, their velocities are a bit optomistic they suggest a 30" barrel can shoot a 140gr. at 3500 fps. I have extensively tested the 6.5 STW(essentially the same cartridge) a 140 gr bullet could be pushed maybe 3300fps out of a 28" barrel(65000psi max). I believe the value of 3200fps for a 142gr matchking is a reasonable velocity and probably runs around 55-60000psi considering the 31" barrel.
There are at least two threads I know of that deal with this topic. The first is in "share your favorite loads" forum where I have posted data for the 6.5 STW with peak strain measurements. The second is a thread dealing with selection of one of these large capacity 6.5 cartridges.The initiating author was GEG. I think this was the gunsmithing forum. There are also other threads asking what is the best 6.5 cartridge. Just do a search.
Have fun!!!
I would think that a Weatherby Accumark in this caliber with an ultra long and narrow bullet of 160 grs be pretty good for hunting.