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To fit an M7 bayonet on a 16-inch AR-15 barrel, you have two main options: install a mid-length gas system barrel or use a bayonet lug adapter. The M7 is historically designed for a 14.5-inch or 20-inch barrel using a standard carbine or rifle gas system. When mounted on a standard 16-inch barrel with a carbine-length gas system, the front sight base is too far back for the M7's ring and latch to reach.

To mount an M7 bayonet on a 16-inch AR-15 barrel, you need a barrel with a mid-length gas system and a standard Front Sight Base (FSB). A traditional carbine-length 16-inch barrel has a gap that prevents the M7's muzzle ring from reaching the lug. If you already have a 16-inch barrel with a carbine gas system, your M7 will likely fall short of locking on properly. You can resolve this without replacing your barrel by adding a bayonet lug extender or a muzzle ring sleeve adapter to bridge the gap.

Hi George and welcome. A couple of options: https://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_ID=266 --------- Cheers, stay safe and enjoy!