Ultimate Arms Gear 1/2"-28 TPI Muzzle Alignment Stainless Steel Shim Set For Threaded Barrels 1/2"-28 TPI for most 5.56mm and 6.8 cal rifles. Shim Kit Is For Mounting Devices. Helps With Timing Of Device 1 shim is 1.5 mm (.06"), 3 shims are .15mm (.0055"). 4 Shims Total. USA MADE