If it isn't SAAMI then isn't it by definition a 'wildcat'? ... even if using a SAAMI reamer, as soon as it is taken deeper it is no longer SAAMI right?
Just my thoughts on this - appreciate you have challenged me to think/rethink about and through it ...
- a special die for full length sizing will be required if the chamber is deeper than the SAAMI in order to shrink the body near the base as it grows over time
- other 'wildcat' cartridges (i.e. 260 rem AI) can often use standard dies except for body or full length (i.e. Lee 260 collet neck and seater for 260 AI) ... design and function of the die will drive this
- 30 vs 40 degree shoulder for improved feeding is arguable but I do agree the 'potential' exists for improved feeding with 30 vs 40 ... there is also a 260 Improved 30 deg shoulder and if not mistaken also a 35 ... and all the above applies to them as well.
Since the creedmoor case was created off the shorter 30TC case (as compared to 308) in order to have the advantages of the shorter case and subsequent ability to seat the long bullets without intruding into/reducing the case capacity, it doesn't make sense to me to try and "revert" the length back to the 308 case length ... unless you just want to experiment with a wildcat. No judgement from me on that - my 6.5 ackley (wildcat) and 6BR (former wildcat) rifles are my favorites for the range ... And all the above being said - I'm curious who is doing it, to what success, what kinds of muzzle velocity and accuracy is being achieved, etc, etc ...
As far as feeding, I have a .260 AI 40degree that I run with an AICS mag, and I can't see how feeding could get any smoother. Now if you were going to run as a BDL, this could completely change! I just know that feeding is very smooth from my American Rifle Company mags. And as far as custom dies, my Redding 3 die set was $100 on a good sale, but you can get a Redding full length bushing sizer die for around $100 even not on sale, and just use a standard .260, 6.5 creed, or similar seating die and be gtg, and with a proper .260 ai, you would be able to shoot factory ammo in a pinch.
But if you like the 30 degree shoulder but want a cartridge like that, the 6.5 SLR is a great solution, and still uses .260 brass, and also will fit better in a short action, as it gets the 30 degree shoulder from pushing the neck/shoulder junction of a .260 down, lengthening the neck.
As far as the 6.5 cm and .260 ai 30 degree being the same thing just longer or shorter, the 6.5 cm actually has a slightly larger diameter base, as in a properly spec'd .260 AI reamer would not clean up a 6.5 cm chamber without being set back. It is only slight, but they are different.