Measuring neck diameter with calipers can be unreliable. Factors such as a small burr on the case mouth, an incorrect angle of the calipers or excessive tension can lead to inaccurate readings. Improper neck tension may complicate projectile insertion or pose safety risks if the neck is too loose. Our gauge addresses these issues effectively. It can remove minor burrs during insertion and maintains the round shape of the mouth, unlike calipers. This gauge is designed for 224 and 308/310 diameter brass, accommodating calibers such as .223 Remington, 5.56x45mm, 7.62mm, .308 Cal, .30/06, 7.62x39mm, among others. On the 308/310 side, the gauge diameters are .299, .302, .305, .308, with an additional smaller step at .310 for 7.62x39mm. The optimal neck tension for a .308 diameter is indicated when the gauge reaches the second or third stop Apply consistent, moderate force during gauging to accurately confirm alignment with the designated stop point The .224 side operates analogously to the .308, where the case mouth should typically align with the second or third gauge stop, preventing it from extending fully to the handle The case mouth gauge provides an efficient and precise method for evaluating neck sizing and neck tension parameters This is a tool I developed for personal use and it has become indispensable to me If the gauge cannot be inserted, the neck tension is excessive and may lead to seating issues Conversely, if the .308 brass mouth slides completely to the .310 stop, it is too loose to securely retain a bullet For a 7.62x39mm firearm, a .32 Cal pistol or a lead-cast .308, the projectile diameter typically ranges from .310 to .312 inches Consequently, an additional small .310 step is provided for these specifications