From the Bushmaster website...
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"A new 5.56mm/.223 Caliber Muzzle Brake – designed right here at Bushmaster – has now been production approved by the BATF. It features four gas ports and a machine turned ribbed design. It is manufactured of quality gun barrel 4140 steel with a manganese phosphate finish, and has proven to be very effective in controlling muzzle rise. Overall length is 2 3/16”. It can be installed without regard to Port location (timing) and will still be effective, however most are installed with ports positioned at 12 - 3 - 6 - 9 o'clock. Slip-on/taper pinned version for Post-Ban barrels (requires factory installation or professional gunsmithing to drill and install the taper pin). .700" diameter at step-down
An internally threaded “Pre-Ban” model for use with Pre-Ban/L.E. threaded barrels is also available"
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I indexed mine at 12 oclock.