I don't own one, but would imagine it takes a .750" gas block. Why not remove the handguard and measure? From pictures online, looks like it's just 2 screws on the bottom of the handguard near the upper receiver.
.750" is most common and most likely what your gun has. Also seen .625" (pencil barrel / M16A1 profile barrels, probably not this barrel profile), .875" (heavier barrel profiles, more for a target barrel), and .936" (heavy target, not common at all).
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