Si buffer tube needed - Strike Industries AR Enhanced Castle Nut / Extended End Plate

The length of the castle nut causes problems. It needs to be threaded way down the buffer tube or end plate won't won't allow the buffer tube to catch the retaining pin.
It wouldn't thread far enough on my Aero tube. I brought it to a gunsmith and he looked at some of his tubes as well. the only one that will work is a buffer tube that has no flat space after the threads. Looking around, only one tube fit the bill, the Strike Industries buffer tube.
So, it seems like, dropping $59 on a castle nut and plate wasn't enough, you'd have to buy SI Buffer Tube for another $59
I returned it..this is getting too much.