FN has not manufactured a FAL for the US market in decades. Maybe you are thinking of the FN SCAR, which does use a $45 proprietary magazine?<>
The newly manufactured FN "FAL Sporter" uses a different very expensive mag. My older Son has one. Beautiful rifle, but they should have designed it for the regular .308 FAL magazines !
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There are two types of FN FAL mags—Commonwealth or metric, commonly referred to as inch or metric. Metrics have a small beak punched out of the front of the mag. The inch version has a larger tab spot welded/brazed on the front of the mag. Theoretically, you can use a metric mag in an inch gun, but not visa versa.