While FN SCAR 17S magazines are sometimes available you can also convert the FN FAL magazines to fit, and even sand down a Magpul Pmag to work in the SCAR. But many really want is a cross-compatible magazine system.
Handle Defense SCAR-25 Prototype
Gearscout has reported Handle Defense have addressed this by creating the SCAR-25 lower receiver* which will use the Knights SR-25 or (as i usually refer to them) M110 magazines. This should include the KAC Stoner mag., Magpul .308 Pmag, C-Products, and DPMS magazines without modification. The SCAR-25 lower is currently available for preorder for $249.95 at Rainier Arms.
Handl Defense’s website is down but they have a Facebook page.
* Yes, it is referred to as a trigger module in the FN parts list
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