I emailed Bushmaster concerning the possibility of a piston upper. Here is what I sent and the response I received:
There is some rumor going around that you are developing a piston AR upper and plan on converting existing uppers. While I don't usually take rumors seriously, this one seems to make sense, since pistons have become popular in AR's and you have many years expirence with them in your M17S. I would be VERY interested in converting two of my Bushmaster AR uppers to a piston system. Is there any true to this? Thank you.
Sorry, only a rumor at this time. It has been looked into and other projects are taking precedent at this time. It may be done at some time in the future.
I remember a reply like this when I asked about a 7.62 rifle. Who knows.