I have had a .257 AI and now a 6mm AI - 40* shoulder.
My gunsmith likes to cut the chamber slightly short so I get a crush fit the junction of neck & shoulder when fireforming - tight fit, essentially no headspace when fireforming. For subsequent loads I use a shortened shell holder to maintain correct head space.
I use whatever pistol primers I have available and whatever pistol powder I have, When fireforming, I start at 10% of the total capacity load then work up so I get nice sharp shoulders.. Coffee grounds (dry) sound good for a filler vs. cream of whatever. Like, if 60 grains of pistol powder fills the case to the top of the neck, I start with 6 grains, the last FF session used 9 grains of HP38.
Dies are available for popular AI calibers. My 6mm Lee collet neck sizer works with 6mm AI brass. I have a Redding 6mm AI F/L sizer die (stock item) and I use a generic 6mm Hornady seating die. Necks are turned to .268 diameter. My brass is adequately fireformed at chamber pressures of about 60K PSI - this is after COW fireforming using full charge loads with bullets. I would not select any AI cartridge that required custom dies - $$ - just me.
With the 6mm AI the case capacity is increased about 10% resulting in 80-100 FPS increase, case stretching is almost nothing, feeding is excellent in my Ruger M77 MKII LA rifle. COAL is right around 3 inches depending on what bullet I use. I planned on getting a 8 twist 6mm AI on a FN commercial Mauser but gave up on the idea when the intended rifle would not feed dummy 6mm AI rounds because the shoulder hung up on the top collar inside the receiver ring - that rifle was re-barreled to 6mm-06.
I use my 10 twist 6mm AI with 87 grain VMax bullets & H4350, RL16,or IMR4831 against rodents - real good performance - 600 yards and beyond. Many 3 shot clover leaf groups and under 2 inches at 300 - good enough for me.
Just remembered - under 10* & snowing - time for a Dickel & fireplace (gas).