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Don't do that. Ask your gunsmith and he'll tell you what to use. But you don't want to do something that might damage your brand new firearm. You might even have your gunsmith work up your AI loads for you so you'll have an idea of the maximum charge you can safely use in it. But your main gain in Ackley Improving is brass life.
They make good barrels and that's a good twist for the heavier bullets. Just work your loads up carefully, about half a grain at a time and monitor the brass for pressure and you should be ok. You can fireform your brass from factory loads if the chamber is done right, which means you can just reload 270 winchester with moderate hunting loads, go to the range, practice with your new rifle and eject a 270AI casing. Then you might want to anneal the neck before reloading just to keep from having the neck crack. If you measure the overall case length on your AI casing, it'll probably be well within length specs, though. The fireforming will usually shorten neck length slightly. Also, trim the case mouth after sizing, just to square it up. It probably won't be over maximum length, but the trimming will square the mouth if it is uneven. I have to square the case mouth on all my .35 Whelen brass that I make from 30-06 casings, which is about 2/3rds of the Whelen brass I fire. But I only have to do it once, when necking up.
My lapua 1st firing in 223 is currently my most accurate load. I'm still early in development, so I hope to beat the factory brass-LOL
LOL, I did not say it couldn't be done, I have just seen it go the other way.
I copied this summary from an old Outdoor Life magazine article: "A wildcatter in Idaho back in the mid-20th century created a line of wildcats from .25- to .30-caliber based on the .30-06 case. Rocky Gibbs took most of the taper out of the cases, pushed the shoulder forward, sharpened it to 35-degrees, and necked it to caliber. Most innovative was his "front ignition primer" via a flash tube extender that delivered the flame toward the front of the powder column, ostensibly for more efficient burning. Mr. Gibbs advertised his 30 Gibbs as the "world's most powerful .30-06, driving 180-grain bullets as fast as 3,100 fps. Independent tests have shown that, when loaded to equal pressures, the Gibbs added just 104 fps to the velocity of the .30-06. Mr. Gibbs, however, touted his cartridge design as contributing to minimal bolt thrust, so traditional indications of excessive pressure, such as a sticky bolt lift, weren't evident. His cases were sold and loaded without the bothersome internal flash extender tube, too. Gibbs claimed 3,175 fps for his .270 version spitting a 150-grain bullet." My brother chronoed dad's 270 Gibbs using Herters Round nose 150gr bullets at +3000 fps. The Gibbs wildcats where really popular in the 1950's around the western Montana home where I grew up. Working class guys couldn't afford the early magnums in expensive guns like the Weatherbys. There was lots of inexpensive military surplus brass and 4831 powder (I think dad paid about 50 cents/lb for 50 lb kegs still in the military containers). Dad's hunting party carried 270 Gibbs, 30 Gibbs, 6.5 Gibbs, or 8 MM Gibbs. All those cases were hydraulically formed that created the short neck case with a rounded shoulder form 30-06 brass. They then loaded ammo at minumum loads to fire form the cases, producing the classic sharp Gibbs shouldered cases. I attached a photo of the Gibbs sizing die with the hydraulic accessory fyi. There should be lots of 1903 Springfieds (cost about $17 from the Civilian Marksmanship Program) sporterized along with some fine old Pre-War Model 70's floating around where I grew up. Probably collecting dust because nobody understands what they are!!
For the .30 Gibbs, I was pushing the 190 Berger VLDs at 2806 FPS with more room to spare and powders I had already opened. I have not settled on a load, considering loads lighter than 180g. I did not chrono the .338 Thor as I am only fire-forming brass, with 18 shots fired (I have 80 more to do), barrel break-in, and sight-in the scope.
You should find a mentor and learn to make fireforming work for you. Also, if you are rebarreling to 6mm AI continually, fireforming brass at the end of barrel life makes sense and can be done quick with a water source and cheap components.
LOL! Dave has been my go-to gunsmith since 2003.
I buy several boxes of factory ammo in the parent caliber of the AI cartridge in the brand I prefer and shoot it. It is always as accurate as any reload I can make, usually, and I don't waste a bunch of time messing with forming brass.
True AI chambers will not have a "headspace problem".
This is not crazy, the Ackley Improved chamber as designed to shoot factory ammo of the parent cartridge for fire forming.
