Pistons run as a minority of the ARs. The idea was to solve all the problems with the GI (did Stoner intend for that to be a pun?) system. Of course pistons just bring their own problems, with added cost. No Free Lunch. One of the costs, as you're discovering is availability of parts.
But on the other hand, you and son now can spend many an hour researching and finding parts together. All isn't lost.