As we see (or hear) on a daily basis, Accuracy International rifle nomenclature can be quite confusing to those not infused with it every day. Below you will find some helpful information to understand the major (and minor) differences between the available and recently discontinued versions.
AT Rifle – This is the AW (Arctic Warfare) replacement and has a more conventional pistol-grip stock design (no forend tube like the AX rifles). This rifle is only available as a short action and in 308 caliber, but barrels are able to be quickly changed by the user via a 4mm hex key. Alternate calibers can be anything that uses a 308-boltface and fits in the AW style #0726 magazine. (243, 260, 6.5 Creedmoor, 6.5x47, 7mm-08 etc)
AX308 (short action), AX338 (long action) – These are the pre-2014 versions, and barrels are removed/swapped conventionally with a vise and wrench. These non-quick-change barreled versions of the AX also lack the skeletonized buttstock that is apparent on the most current versions. The forend attachments use counter-bored key-slots (squared-off at the bottom of the cut), vs the countersunk (tapered head) variety in all other current versions (AT, AX308, AXMC) – the countersunk modification was done to better secure rail-attached items during impact for more critical uses like laser aiming devices.
AX308, or AX Short Action (yes, the same label as the rifle above, but manufactured from 2014 onward) – These are also short action rifles but are equipped with the quick-change barrel setup utilizing a 4mm allen wrench (that is actually conveniently stored under the cheekpiece). The most obvious way to identify these quick change-barrel versions would be the visible head of a cap screw just ahead and below of the ejection port under the serial number. If your rifle does not have this, then you will need a vise/wrench to change the barrel. These rifles also have the obvious skeletonized fully adjustable butt stock that it shares with the AXMC.
AXMC – This is the true multi-caliber (multi-boltface) capable platform that can span from 308-sized rounds up to the impressive 338 Lapua. This rifle starts as a magnum caliber (currently only 300 Win or 338 Lapua), then you are able to convert and swap among the 3 major bolt face sizes (308/0.473, magnum/~0.540, Lapua/0.590). For the 300 and 338 magnum calibers all that is needed to switch caliber, other than the proper barrel, is the appropriate magazine and bolt body. When switching to the short action 308-boltface rounds, the magazine adapter is also needed to bush down the magwell to fit the standard AX rifle 308 magazines. Conventionally the main firing-pin assembly is shared among the 3 possible bolt bodies, but for those that wish to have complete bolts ready to go, they are available to purchase as a separate item also (replacement firing pin).