Re: .25-06 AI fireforming question.
BillR, I concur with the advice of both WildBill, and CentrePunch.
I currently have two rifles chambered for the 25/06AI, and both have a chamber neck diameter of 0.291. I am certainly glad that I don't have one with a .282 chamber neck, as you would have to love neck turning to have one that small.
I don't believe that factory 25/06 loads would chamber in your rifle due to the tight neck.
I can understand someone wanting a .286 or .288 chamber neck using the 25/06 or AI, but it is hard to understand the rationale for having a chamber neck that small. To achieve a neck diameter of .279, you will have to reduce the neck thickness on either Remington or Winchester brass from about .014 to .016 to a maximum of .011, which in neck turning terms is quite a lot.
Using 25/06 brass will be a bit easier than .270W brass, as you will not have to trim about .040 from the necks before loading for fireforming. However, using 270W brass does allow you to start the brass at the chamber length of 2.494, and after fireforming, the case would end up at about 2.485.
New Remington or Winchester 25/06 brass seems to be about 2.485 - 2.490, and after fireforming can end up between 2.470 and 2.480. However, to me that is no big deal, and it just means that you will go an extra 2 -3 shots before trimming.
I always neck anneal "improved" and "Wildcat"cases after fireforming, and have found that the necks on new Remington brass are far better annealed than new Winchester brass. As a result at least 90% of Remington case necks expand correctly when fireforming, while at least 90% of Winchester case necks don't, when fireforming in my 22/250AI and 25/06AI.
The capacity of the Winchester brass is usually about 1 - 2 grains greater than that of Remington brass. However, although you may use 1 grain less powder with Remington brass, I have found that velocity achieved from either is about the same.
You don't mention what game you intend to shoot, and what bullet weights you are considering using. Due to the wide variation in chamber dimensions, it is inadvisable to give someone else one of your own "pet loads" for use in his rifle.
However, if you let me know what bullet weight(s) you intend using, I will suggest which powders are the best options.
I agree with Wild Bill, and suggest that when fireforming you choose a maximum 25/06 load for the bullet weight you choose. Hope that the above is of some help. Brian.