The FN America SCAR17S is one of the best battle rifles out on the market. It fires a 7.62x51 cartridge but the recoil is oddly soft for a .308 winchester rifle. The short stroke gas piston system allows the gun to keep on firing in all manner of conditions. Even though it only has a 16.25" barrel witn 1:12 twist, it is accurate out to 800 meters.
FN offers magazines in two different capacities. 20 rd mags or 10 rd mags. The SCAR17S magazine is actually a modified FN FAL magazine. The SCAR17S and FAL Mag body is the same. FN just cut a mag catch notch into the side of the mag body, removed the front notch and changed the feed lip shape a bit. Then they added a plastic bumper to the base plate.
These mags are great but they are a bit pricey and they weigh a lot. Empty, the factory 20 rd mag weighs 8.6oz.
For a lighter option there are aftermarket polymer magazines. One of the first to come to market was Molon Labe Industries. They made a universal polymer magazine that they can cut to different lengths and have different capacities. Regardless of size the magazines are all the same price. They come in 5, 10, 15, 20, 25 and 30 rd capacities. I have had some issues with reliability with the 25 rd mags. Fail to feed. Also when loading the rounds into the magazine, the magazine tends to swell a bit and does not drop free if you need to fix a malfunction. They are lighter than the factory magazine though. A 20 rd mag weighs 6.1oz and the 25rd magazine weighs 8.1oz. One thing nice about the Molon Labe Industries magazines is their giant base plate. It is perfectly flat and a little wide so it acts like a mono pod.
Another contender in the polymer magazine category is Moses Mag. They started off by making polymer magazine for FAL pattern rifles. And since the FAL magazine is not that different from an FAL magazine, it was not that difficult to alter the mold and make a polymer magazine for the SCAR17S. The magazine is the lightest of all the SCAR17S compatible magazines. Empty it weighs a mere 4.6oz. That is almost half the weight of the factory FN magazine. The Moses Mag SCAR-H magazine holds 21 rounds of .308 winchester. However if you load it to a full 21 rounds, you will have a difficult time inserting it on a closed bolt. So it is best to leave it at 20 or open and lock the bolt for the full 21.
The final magazine is basically an extended metal magazine similar in design to the FN magazine. ASC makes this long metal magazine and it holds 25 rounds. Weight wise it is not that bad at just 8.1oz. Still lighter than the factory magazine. Both the ASC magazine and the Moses Mag magazine have sloped base plates. While you can monopod the rifle on the magazine, all the weight is concentrated on that one narrow section of the magazine. Both magazines are equally priced at below $30 each.