In the AR platform you have a bit of micro wobble. In the .308 with a polymer lower it is like firing a 155 Howitzer on top of a matchbox car. That micro movement of a millimeter back and forth per round wears those rear take down pin holes with the bolt and buffer rocking forward and backwards. Secondly the heat created during milling changes the density of the polymer. The buffer tube threading is maybe just installing the buffer or could also be from when the spring is compressed it is creating a bind point that causes the weakest point to flex. The aluminum tube will not flex, the soft polymer will.
