The Ruger AC556 stock, and folding stock, have additional cuts to accomodate the wider AC556 receiver. Remember the AC556 and Mini-14 receivers are fundementally different, so yes, the stocks are different.
The good news is that, if you know what you are doing, you can modify a mini-14 receiver to accomodate an AC556 receiver. In fact, there is a guy that does this to buttler creek folding stocks, so I know it can be done.
Most people who have AC556 folders would never sell off their folding stock, even if they mounted the receiver in a different stock. I have one, put it up, as I have mine mounted in a SCAR-CQB stock, but would never part with the original folder.
Personally, I'd spend the $200 more and get the SCAR stock from Clyde Armory. You can then put whatever kind of buttstock, folding, colapsible, or fixed you want on it. You can also mount any accessory that typically mounts on standard rails. The stock is all aluminum and much more solid than any of the wooden stocks.
