Axis barrels have the same thread as a standard shank. The "ball bearings" you see are actually small steel shot used to create the matte finish. It is common for those to accumulate in the thread joint around the front of the nut and be difficult for removal of the nut. Try flushing out with some carb cleaner. Use a wire brush on the inside threads of your receiver to remove stubborn shot that is wedged between threads. One of the small stainless steel brushes used on copper pipe fittings works excellent.