The seven-lug bolt has a deeply recessed face with a plunger ejector and a claw extractor. There are a number of mechanical features that help ensure the 17S’s multi-lug bolt seats consistently into battery. The mass of the bolt assembly combined with the mechanical advantage of the SCAR’s fixed charging handle eliminates any need for a forward assist plunger.
As one can imagine, these improvements greatly simplify the immediate action drill. In the event of a stoppage, there is no need to tap a separate forward assist after you pull and release the charging handle. Also, there is no charging handle to pull over the top of the stock, so one can keep his or her head on the stock when reducing a stoppage to get the gun back into action that much faster. The stock also has a two-position adjustable comb. Given that many of the powerful optical sights currently available require the use of tall rings and bases, this is an important feature.
Its six-position collapsing stock is indexed on both sides. Compressing a metal release bar on the left-hand side of the stock allows the user to adjust length of pull, and depressing a half-moon-shaped button at the wrist folds the stock to the right side of the receiver. A stud on the comb locks into a hook at the rear of the ejection port that doubles as a shell deflector. The hook is just a friction lock, so a sharp pull on the butt of the stock will release it from the hook.
The SCAR 17S has a number of sling attachment points. At the stock’s wrist are two vertical sling loops on the left-hand side and one on the right. Two more vertical sling loops are fixed to the mouth of the fore-end. Last, a horizontal slot runs through the stock’s heel.
A 37-position M1913 Picatinny rail runs the full length of the aluminum alloy upper receiver. The rail runs all the way from the gas block to the wrist of the stock. The mounting points are indexed so it is easier to quickly reinstall optical sights without altering zero or eye relief. Also there is no slip ring on the one-piece upper receiver that can create steps and gaps to complicate scope mounting.
The polymer lower receiver has a larger magazine well to accommodate FNH USA’s proprietary double-column, detachable box magazine. The magazine, of which 10- and 20-round variants are available, has a stamped steel body and a polymer follower, and the floorplate has a wedge-shaped extension that gives the magazine the appearance of having a flat bottom.
Detachable flip-up iron sights are included. The rear aperture overlaps the circular shroud for the front sight—a system first seen on the G3/H&K-91 that aids rapid sight alignment. The front post is detent-adjustable for elevation. Vertical drums located on both sides of the rear sight base provide adjustment for windage, and a horizontal wheel at the foot of the rear aperture indexed from 200 to 600 meters allows adjustment for range.
The cold-hammer forged barrel has a moderate profile. It measures 0.66 inches in diameter and is 161/4-inches long. The twist rate is 1:12 inches. Fabrique Nationale chose a four-prong triple-baffle muzzle brake from Primary Weapon Systems. The free-floating barrel is attached to the receiver via six Torx-head machine screws. Swapping out a barrel requires a Torx wrench with a proper torque setting and takes about five minutes. FNH-USA states that when SCAR barrels are swapped, loss of zero should be limited to less than 1 inch at 100 yards.