If you neck down .30-06 brass...
You'll need to do some inside or outside neck turning. Otherwise, you'll have exceptionally thick brass in the neck area, and that will cause excessive bullet tension and higher chamber pressures, something you don't want.
Luckily, there's several other options, to include necking down .280 Remington, .270 Winchester, and necking up .25-06 brass. Of course, all of these need to be trimmed to proper length after resizing.
If you're real resourceful, you can get a batch of either 6.5x64 Brenneke, or 6.5x63 RWS brass, and just fireform it. Those are the European equivalent of the 6.5-06. There's also the 7x64 Brenneke, which is a higher-pressure European version of the American .280 Remington. It makes a fine parent cartridge to form 6.5-06 brass from, too. I use RWS 7x64 brass for my own 6.5-06 loads, Huntington's used to have a 55-gallon drum full of it in Oroville years ago.
A-Square legitimized the 6.5-06 a few years ago, and I *think* they still offer properly-headstamped brass for it. I understand A-Square's move chapped Remington's hide, because they wanted to offer the 6.5-06 as a factory round and standardized 700/40X rifle option.
The beauty of the 6.5-06 over the .270 is the variety of high BC (Ballistic Coefficient) bullets in the 6.5/.264" diameter. For some reason, the .270 has always remained a hunting round, and never really caught on as a target or 1000 yard match cartridge, so there aren't as many bullet offerings compared to the 6.5mm, either here or overseas. That's a shame, because it wouldn't take much for an enterprising bullet manufacturer to design and produce a decent, high BC, long-range .277" bullet, either in a hunting or target configuration. Sierra has their MatchKing, but it's rather ho-hum, with a .488 BC.
Until somebody does come up with a better .277" bullet, the 6.5-06 maintains the ballistic advantage out to 1000+ yards.
