I recently purchased a 6.5-300 Weatherby and also have a pile of unused 300 Weatherby brass I would like to convert to 6.5-300 if this can be done fairly easy.
You most likely will need to anneal the case necks first if the cases have been fired before. If they are new you should be able to use a good wax type of lube and run them into a full length size die setting it up to cam over. It will help if you have a 7mm die to run them in first to slowly step them down. I have found that in reforming cases that using a quick full motion on the size works better than a slow motion. I make my 264 Win Mag cases out of once fired Winchester 7mm Rem. Mag cases. By the way the AccuBond bullets work very well in these screamers. I have a 27 3/4" barrel on my 264 Win mag and run the 130 AB 3350 fps. Have shot deer from 25 to a touch over 500 yards and bullet exhibits the same performance. Goes in takes out the vitals and exits with a nickel size hole and deer are DRT.
To piggy back on the original posters question (hopefully not hijacking), I need to convert 28 pieces of new 300SAUM cases down to 7mmSAUM. So, can I just neck size, trim, and turn necks?