The Ballistic Tools case mouth and neck tension gage tells you exactly how much neck tension your reloads will have.
Measuring neck diameter with calipers is an iffy proposition at best. A small burr on the case mouth, a slight angle to the calipers, or even just placing too much tension on the calipers can give distorted readings. Improper neck tension can make projectile insertion more difficult, or even cause a safety issue if the neck is too loose.
Using our gage solves all those problems. It can knock off a small burr during insertion, and will not distort the round shape of the mouth like calipers will.
This gage is for 224 diameter and 308/310 diameter brass. It will do calibers such as .223 Remington, 5.56 NATO, 7.62 NATO, .308, .30/06, 7.62x39, and many more.
On the 308/310 side, the gage diameters are .299, .302, .305, .308, and a smaller step at .310 for 7.62x39.
Ideal neck tension for 308 diameter would indicated when the gauge slides to the second or third stop. If the gauge will not insert, neck tension is excessive and may cause seating problems, if the .308 mouth brass slides all the way to the small .310 stop, it is too loose to retain a bullet. Use moderate pressure when gauging to ensure you are on the correct stop.
For 7.62x39, 32 caliber pistol, or lead cast .308, projectile diameter is often .310-.312, which is why there is an extra small .310 step for those.
The .224 side works the same as the 308, in that the case mouth should generally gauge to the second or third stop and not slide all the way to the handle.
This case mouth gage is a quick and reliable way to test your neck sizing and neck tension. It is a tool I developed for my own use and I could not do without it. Order yours today.