HDM-A2 13 ARCA Rail

The HDM-A2 13 ARCA Rail is a free-float HDM-A2 handguard machined from billet aluminum and finished in black Type III hardcoat. It carries a continuous MIL-STD-1913 top rail that indexes flush with a flat-top upper, and an integrated ARCA-Swiss dovetail along the bottom, plus M-LOK at 3, 6 and 9 o'clock.

CMT's barrel nut does not have to be timed. There is no shim stack to chase and no clocking the nut to clear the gas tube — torque the nut, slide the rail on, and clamp it. That removes the single most common source of a canted rail and lets the same nut come back off for barrel work without a fight.

Fitment

• Mil-spec AR-15 flat-top upper receiver
• Intended for a 16 in. barrel running a mid-length-length gas system
• Clears low-profile gas blocks up to .750 in. journal diameter
• Will not clear a standard A2 front sight base or a clamp-on gas block with an oversized body

In the box

• HDM-A2 13 ARCA Rail
• CMT barrel nut
• Clamp hardware and wrench flats sized for a standard armorer's wrench

Installation. Fit and torque the barrel nut to the value in the instructions with the upper in a proper vise block and reaction rod. Confirm the gas tube passes through the nut without contact before clamping the rail, and verify gas-block alignment before the first live round. If you are not equipped to torque a barrel nut, have a qualified gunsmith install it.