They are available in the UK. Basically a side charger action with the action spring removed.
The basic concept of the Stoner design was to use the barrel extension to lock the bolt lugs into, allowing the receivers to remain unstressed and removing dead weight. Changing that to a true Mauser action would mean replacing the barrel extension and upper receiver with a heavier steel receiver that the barrel would be screwed onto, and the bolt lugs into. It would also mean assembling them and then checking for headspace, with whatever more complicated measures were necessary to get it right.
With the Stoner design, the barrel extension simply screws on and it's headspaced like a micrometer, simply turned until it's right, and then pinned to hold it. Huge contrast to the old school method of a multi-ton hydraulic press and hours getting it right.
If anything, an AR action as a manual bolt would cut the receiver length by half, it only needs a carrier long enough for the bolt, not the extra dead weight added behind. It could be cut off right at the gas rings. We'd wind up mounting the rear optic on or over the barrel.
