There is a newcomer to SCAR17S magazines. Moses Mag makes polymer 20 rd magazines here in the US. Previously, the only other polymer magazine, for the SCAR17S, was the Molon Labe Industries magazines. The Molon Labe magazines have inconsistent performance. General consensus is they work, but are not as reliable as the factory magazines.
Moses Mag sells their magazine for $27.99. Moses Mag, started making polymer FAL magazines. For those not familiar with the SCAR 17H/S, the magazine is a modified FAL magazine. The feed lips are slightly different, and there is a side magazine catch notch for the SCAR mags. You can actually modify FAL mags to work in a SCAR17 with a little dremel work.
They posted on their website on 6/13/2015 that they completed the injection mold. They say that their polymer magazines are 40% lighter than the factory metal magazines.
I like the baseplate design. It is a 7.62×51 round that looks like it is traveling thru the air. Slight issue, is that it is a complete round and not the projectile but that is just nitpicking. It is a cool design. From the looks of the baseplates, they will be molded in OD Green and FDE. But at the moment, only black magazines are for sale.
Here is a photo of the SCAR 17 magazines right out of the mold. You can see two are still conjoined by the molding sprue.
Here is a video of them test firing the magazines.