This gage will allow you to check whether the 1/2-28 threads on a 5.56mm weapon are concentric with the bore. Most sound suppressor manufacturers list 0.003" (3 thousandths of an inch) as a standard tolerance for thread concentricity. This tool will allow you to see if your barrel is within this specification. If the pin does not slide freely into the barrel, this means your threads are out by at least 0.003”. This could cause a sizable point of impact shift and possible baffle strikes.
If the pin does not slide freely, it is advised to have your barrel inspected by a competent gunsmith.
This device is not an absolute guarantee of concentricity.
The pin is made of O-1 tool steel, hardened to 50 Rockwell C, then ground to size. This pin is ground to 0.2178. It should fit most all 5.56 rifling. The gage body is made of 4140PH and Parkerized.
*The pin is made of steel and will rust if not maintained with a light coating of oil.
*Will not fit .22 rimfire barrels.