Sadly GD, I can't find anything that fits the M3 or Beretta shotguns. a buddy of mine has this on his 870,

http://mesatactical.com/products/leo-telescoping-stock-adapter-for-rem-870-12-ga/

I can't imagine it would be difficult to fit a Leapers AR folding stock adapter to it. I was happy with my Leapers, until the good idea fairy struck with a dremel tool... but hey, for 10$ at the time, I wasn't too upset.

http://www.leapers.com/prod_detail.php?itemno=TL-K7FAD01