Curt, the rifles were dragged a couple hundred feet.
Thekatar, this test was originally invented by the glock worshippers to demonstrate how rugged their pistols were. Carter bet that an AK could stand up to the same test a few years back and was proven right. Since a chicom or other decent AK could stand up to the test, we had questions about the $1000 Krebs gun (well $1200 with the Ace stock, sling, and transfer fee), this effectively answered any questions we had about its durability. Surprisingly there was little damage to the finish of the rifle, not even a scratch on the receiver.
BTW, we did shake as much debris out as we could after we removed the test mag and then cycled the action a couple times, we're not crazy after all, a rock in your barrel will ruin your day no matter what gun you have.
If there was a scientific reason behind the test, I'd say it simulates a rifle getting excessive debris/dust/dirt/sand in the action, or falling off a moving vehicle.
Yes there are different kinds of harsh tests, like freezing a gun, firing 20,000 rounds out of it, dunking it in mud and using proof loads. However for AZ, this is the test that proves whether a gun can function or not. If you note in the last picture, AZ earth will make a rifle look like it came back from the sandbox in short order.
We have yet to have anyone volunteer an AR-15 type for this test, or the simple version of this test (tossing a handfull of dirt in the receiver and shaking it out).