IIRC, they work, however they add considerable weight right at the muzzle where it does nothing good for the handling qualities of the rifle. A less expensive alternative is to bed the action into a tight fitting synthetic stock, and affix a John Masen "Black Warrior" muzzle brake/comp. Go here:www.perfectunion.com/forums/index.php?showforum=55, and start reading/asking.
"The M1 Rifle is the greatest battle implement ever devised." General George S. Patton Jr.,US Army