High speed light rail.

"I just can't get past the fact that they're full of... of... toaster parts." -Bob Howard

FN FNS-9 on the left, Walther PPX on the right. The FN has a pretty beefy "chassis" comprising the front frame rails and locking block all in one piece. The Walther has very Glock-like vestigial tabs for front rails and a separate large "axle" for the lower barrel lug to cam against, a la the 1911 slide stop pin.
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