Les Baer Mid-Length Monolith .308 with a 16-inch barrel. JP Rifles LRP-07 with a 16-inch barrel. Knights Armament SR-25 with a 16-inch barrel. LMT .308 Modular Weapons System with a 16-inch barrel. LaRue Tactical makes several .308's with 16-inch barrels.
Then there is always the FN SCAR 17S with a 16.5-inch barrel. Not an "M-4" configuration, but is a .308 with front rails.
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