Re: LMT piston 16"
Threads are 1/2x28 (standard AR-15/.223/5.56 threads).
What are your goals for the muzzle device? Do you want a true muzzle brake to reduce perceived recoil/muzzle flip or do you want a flash hider?
The Vais brakes are very effective for reducing muzzle flip/recoil, but they are loud as hell and increase flash. They are also a real PITA when it comes to shooting prone unless you have something covering the ground when firing. Because the ports in the brake are 360 degrees, you get a lot of dust/dirt kicked up with the Vais.
Other good muzzle brakes include: JP Tactical Comp/Bennie Cooley Brake and the PRI/MSTN QC Brake are some of the better choices...but really, the options are nearly limitless these days.
