BjornF16, I can answer your question as I own one also: The FCM is set up for a standard short action detachable mag. The 7.62x39 mag that comes with it has an overall internal mag length (because of a spacer that Savage puts in the rear) of approximately 2.46 inches.

You could easily replace that mag body, spring & follower with a .223 mag body, spring & follower or a standard short action .308-family mag body, spring & follower if you so desired. If you do, there are those (like me) who would likely be happy to purchase your newly-redundant mag body. :D