The SOLGW NOX muzzle device is THE answer for getting your 13.7" barrel up over 16", while simultaneously enabling mounting any Keymount compatible suppressor.
The QPQ treated muzzle device features two sets of ports designed to time for right hand use at 12 o'clock and 3 o'clock; or rotated 90 degrees further for left handed uses to port at 9 and 12, instead.
Forward of those ports are the signature trio of open tines, maximizing flash reduction without any unwanted high pitch ringing common with other popular open tine flash-hiders.
Shims are included with this 1/2x28mm muzzle device, and it is pre-drilled for pinning and welding (3/32" hole) to any barrel 13.7" or longer to bring it up over 16" in length.