I ran a 264 WM for a number of years. I'd be surprised if you reliably hit 3300 with 140's. Neither of my two 26" barrels hit over 3200 with Retumbo or 7828. If I were to have a 264 again I'd check out R26. It's my go-to powder for every caliber I own (22, 6 and 6.5). It must be made from unicorn hooves and horns.
There's actually quite a bit of data out there from guys running them that support the 3300FPS figure, plenty have done it. Some have even seen 3400FPS in longer barrels. Regardless, I'm after the magic 3150fps which will certainly be achievable with a very reduced load and/or shorter barrel.
I've got no real interest in running anything but Hodgdon Extreme powders and not have to chase velocity with temp changes. I've done that one too many times and don't stray away from the extreme powders unless I absolutely have to. I generally give up some velocity as a result to keep the consistency.
I'm running a 6.5-270 WSM in two different long actions. For bottom metal, I'm running Rem BDL and the other is Curtis Customs with their magazine. They had some mags with longer feed lips and I've had zero problems feeding. The BDL took some tweaking of the follower but it reliably feeds. I'm hitting 3200 with 140's with a number of different powders with either one. I was getting 3420 with 130's in the one rifle I tried 130's.
You don't need custom dies for this round. Use a 270 WSM bushing die and swap out bushings. Sizing is done in one pass. I'm using a Wilson chamber seater but you can actually buy a 6.5-300 WSM die set from Redding.
All three 6.5 WSM's I've run have been wicked accurate with very little load development. My 264's took a little more work.
I've built two 6.5 SAUMs for other guys (and have a barrel I've fitted but not used for myself) and both fed reliably in a short action. One used the Magpul Hunter stock and the plastic mags and the other was a Manners mini chassis with AICS mags. I haven't tried them in a long action
The issue with all of this is we're talking about an AXMC that uses either a double stack double feed LA magnum magazine or a SA double stack double feed AW or AX mag with the adaptor housing. This changes things completely. I highly doubt that a LA double stack mag is going to feed reliably with the rounds jumping that far to the chamber. The SA double stack rounds are completely unproven with the SA magnum rounds, everyone I know who has tried it had problems and it's probably a reason why GAP sets up AICS style mags for their 6.5SAUM's vs modifying AW mags.
6.5WSM would be a viable choice but for these rifles it falls right into the same issues as 6.5SAUM.
I relayed this idea to a friend of mine with a TRG- he ran with it. We split the cost of a reemer. I'll snag a barrel and move over to it next year.
I'm waiting on word back, but if I recall correctly, he's running 21", retumbo, and pushing 140 ELD to around 3100fps. Given PRS velocity limits, this makes one hell of a potent combination in a short package.
Barrel life is not much concern since he's finishing the blanks; cuts cost by about half.
Who did you order the reamer from? Is it a standard 264WM or custom dimensions? What length throat did he spec?
As far as load, how high did he work up and did he find pressure? Do you know what the max velocity was that he saw with what charge? What barrel did he go with?
I was planning at starting at 28" and doing load development from there. Once I get everything in order we could compare data from both and probably figure out +/-50 FPS different barrel lengths will see with a mild charge.
I'm not as concerned with barrel life from a cost perspective as I am from a PITA perspective. Each barrel I've got to tweak the load to it, then after a couple hundred rounds it decides to speed up a bit, have to spot that and make sure everything is still good to go, and then I'm to prime time for shooting until it slows down. If that's in 800 rounds total then I'm not interested. 1200 rounds minimum gives 300 rounds to break in/speed up and get the load developed and then a minimum of 3x 300 round count competitions. That's what I need it to get to be worth my time and I think it's easily doable. This first run will be entirely trial and error and I really have no expectations one way or another, just hopes.