Polymer magazines are not a good fit for the SKS M in the original stock. While a PMAG can be forced into position, usually after a few tries, it certainly isn't a natural fit and the magazine is slightly warping inwards to make it happen. The stock is the limiting factor, as when you pull the receiver out of the stock, the magazines fit perfectly. Modification of the magazine well will allow for use of polymer magazines but seems much smarter and easier just to get metal magazines since they are plentiful and work well.