Bryan, AI means Ackley Improved. P.O. Ackley was a famous gunsmith that discovered if you change the chamber's shoulder angle, the barrel's throat will last a lot longer or you could run hotter loads. The modification to the gun is nothing more than running a special made reamer in the chamber. Of course you must fire form your brass with light loads then use a special 25-'06 AI sizer die when reloading.
Some folks say it improves accuracy but it really doesn't unless you were doing something wrong with the standard 25-'06 loads to start with. The Ackley improved shoulder has been applied to many different cartridges that are known barrel eaters. Actually "throat eaters" to be more exact. You can accomplish the same affect by using a powder charge that is slightly reduced, typically 5% less than factory standard loads. Of course you will reduce velocity whereas with the Ackley improved shoulder angle, you can shoot full velocity loads and enjoy a longer barrel life.
I was very fortunate to meet Mr Ackley many times back in the early '70s. He was often a guest instructor at the gunsmith school in Trinidad, CO, which he founded. P.O. was the best "wildcatter" in the world plus he was a very accomplished rifle smith. During one class, he explained how the neck of the cartridge acted almost like a lens in a camera to focus the pressure on the base of the bullet. By optimizing the angle of different cartridges, you could run them with hotter loads or in the case of barrel eaters, improve barrel life without compromising power. He never claimed any improvement in accuracy. P.O. died in '89 so many of the Ackley Improved cartridges you see are not "true" Ackley and many of the claims of accuracy improvements have been embellished over the years.
Bryan, The 25-'06 is a known "barrel eater" so rather than increasing velocity, the Ackley improvement is intended for increasing barrel life (reducing throat erosion). The typical factory load with a 100 gr bullet (optimum bullet weight) runs about 3200 fps from a 24" barrel. With the Ackley Improved chamber, you can run up to 3350 fps with the same bullet (about 5% increase), however running loads that hot will erode the throat just as fast as a non-improved chamber with standard factory ammo.
Typical barrel life with a 25-'06 is about a 1000-1500 rounds. The 25-'06 AI is supposed to extend the barrel life to 1800-2000 rounds, assuming standard velocities.
Bryan, FYI ... any time you have a high cartridge diameter vs bullet diameter ratio, you have a potential for throat erosion. If you compound that ratio with a large capacity case, it increases the risk of throat erosion considerably. Rate of fire also has a big influence on throat erosion.
The 25-'06 has both characteristics so it will be a barrel eater or more specifically, throat eater. The 257 Roberts shoots the same diameter bullet and has the same case diameter to bullet diameter ratio as the 25-'06. Because is has less case capacity and runs about 200 fps slower, the barrel life is much longer (2500-4000 rounds). The worst known cartridge for throat erosion is a 264 Win Mag. Barrel life with full power factory loads is about 500 rounds. Some of the Weatherby cartridges are also short lived.
The worst thing you can do with a known barrel eater is to fire too many rounds in a short period of time. The steel in barrels is pretty resistant to erosion until it reaches 140-160 degrees. This, coincidentally, happens to be about the most heat your hand can stand without feeling burned. So ... place your hand on the chamber after firing. If it is too hot to hold, wait until it cools down. A typical rate of fire would be one round every 10-15 minutes.
As the chamber's throat erodes, accuracy and velocity will start to drop off very gradually at first then all of a sudden the gun shoots patterns instead of groups. Throat erosion is caused by the heat focused on a small area and the chamber pressure driving small carbon particles to the hot area. This affect is like a plasma cutter and sand blaster in one short operation.
The Ackley Improvement will help a lot but it is not a "fix all" for barrel life. You must still keep the velocity down to SAAMI standards and hold your rate of fire to where the chamber doesn't over heat. If it were my gun and I planned to shoot it a lot, I'd back off a couple hundred fps for target loads and save the full power loads for hunting.