i run the Picatinny rail mount Caldwell bag on my ar's. you can buy extra rail mounts for about $15. if you're not running your optics super low this bag fits right underneath it on the receiver section of the rail between 2 peice rings, or even behind a 1 peice cantilever style mount. its adjustable for position as well (there are setscrews that hold the bag side of the mount along two rails that allow it to slide side to side)
there used to be a couple aftermarket rail adapters that would fit (i own several) but it appears they've changed their design, so i cant verify if the new ones work anymore