When purchasing Brownells or any other brand from eGoods365.com store, you can be sure that (a) you are purchasing from US based company with the most responsive and friendly customer service; (b) an authentic, not a counterfeit and not a refurbished product.

There is a very narrow, (.004") throat diameter range where
maximum accuracy is obtained. Go smaller or larger and accuracy
will suffer. By simply slipping the Plug Gauge into the barrel throat,
it’s possible to tell at a glance when you are approaching the
correct diameter. Warns you before you go too far. Also valuable
as a diagnostic tool for guns brought in with accuracy complaints,
(designed for chamfers cut with our tools - may not give a correct
reading on guns with other degrees of chamfer). Each gauge is
self-piloted, precision-ground, hardened and stepped to show
minimum and maximum optimum diameters. 11° and
18° Gauge measures both chamfer angles. Sizes .22RF;
.32; .38/.357; .38-40/.40; .41; .44; .45. 5° Ruger-Style
Gauge available in .38/.357, .44 and .45 caliber
only.