I would first make a list of everyone who has ever made a piston variant of an AR or upper, ever. Next, for each maker, make a list of every variant they've produced. After that, find out who their licensed distributors are, whether they're middle-man warehouses, or retail outlets/sites. This step is the hardest: contact the person in charge of each of these retailers to find out how many they've ordered... and even then, about the best you'll ever get is a ballpark figure going back a month or two, becoming more difficult to quantify the further back in time you go.
From there, you'll have to intelligently fudge figures a bit: how many buy, versus how many of them bought it and then talk about it? Heck, how many talk about it that didn't buy it, but parrot anecdotes to either sound 1) informed or 2) to advance whatever agenda for/against the product? At this point, you're talking about the socioeconomics aspect of market research, which is grad-level investigation. You'll have to start extrapolating at some point, which is a slippery slope for incorrect assumptions (which by this point you may already be on).
Another approach, which is far more tedious, would be to find specific users of piston systems, learn what they use (and tried before in the past), their impressions of them, what other armament they have, and their ratio of discussion of the piston to the standard design. Again, you'll have to fudge numbers, but factor in how many comments/likes/views/etc they have, and then use those numbers as a factor to determine their overall market influence. Negative reviews are a major part as well. Positive reviews reinforce what a potential buyer wants to hear; negative reviews' influence potential purchasers from complete turn-off to "my (situation) is different, so what they posted doesn't apply."
The fudging from all this comes from everyone who buys a product, but never mentions it once. Further fudging comes from those who bought it secondhand.
And, in the end, the best you could hope for is merely a numerical range, not hard numbers. Best of luck - but answering a question like this, that depends on figuring out trade secrets that border on politics, is something that in the end can only be, by definition, conjecture.
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