It is my understanding that if you convert a standard chamber to an improved chamber, one can shoot factory ammo in the improved chamber without a problem. In the 257Roberts I noticed that the distance from the rim to the begining of the neck on the standard chamber is 1.912 and for the 257 Rob. Ackley Improved is 1.906. Also on the 30-06 the std is 2.109 and the AI is 2.060. I know when reaming the chamber for the inproved chamber one can not add metal to the neck to bring it back to the shorter dist and if the barrel is turned in a little for this you probly could not use factory rounds, what am I missing? so what am I missing?

Someone who actually chambers barrels will likely weigh in on this, but I'll give it a shot. An "improved" chamber is a little shorter as you noticed in order to create a slight "crush fit" on factory spec cases. I think this helps insure a good fire-formed case.

To make a AI chamber correctly you must set the barrel back. Normally one turn. The cut the AI chamber. Then factory brass will then be a tight "crush" fit.

I made the classic mistake, I headspaced the 257RAI like a 257R.
The problem is the firing pin pushes the case into the sharp steep shoulder of the chamber .004" and then stretches the case .004".

The right way to do it is do chamber a 257RAI .004" short.

With the handicapped rifle, I have to fire form in a two step process to keep from stretching the brass.

If one wants to make a 257R into a 257RAI, he should set it back a thread if there are open sights, or set it back at least .004" if not.