Fab makes their buttstocks fit both. There is no one or the other, just buy it and slap it on. Look at all their buttstocks along the lower keyway (dont know the correct term) of the buffer tube and you will see fingers that press inward on the buffer from either side to take up most of the slop. The only slop is a little bit of up and down that rotates off the pin. Doesnt mattter which tube you have, the Fab buttstocks fit it.