I think the difference between the 500 and the 500a is the stock / forearm size and the capacity. The 500 being the full size and the 500a being the superbantum I think. Cant say anything about the 500at. Whether or not a 500 barrel will fit a 500a or 500at depends on the length of the mag. My 500 holds 5 down below, and if the bantum (500a) holds the same amount, then I believe the barrels are interchangable, but I could be wrong. My manual covered both the 500a and the 500 though, and there were very few times they were distinguished in the manual.