Case Mouth & Neck Tension Gauge
Case Mouth & Neck Tension Gauge
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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.

This gauge solves all those problems. It will not distort the round shape of the mouth like calipers will, and can even knock off a small burr during insertion.

This gauge 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.

Case Mouth Gauge How To Use The Case Mouth and Neck Tension Gage
On the 308/310 end of the gauge, the gage diameters are .299, .302, .305, .308, and a smaller step to .310 for 7.62x39.

Ideal neck tension for .308 diameter is indicated when the gauge slides to the second (.302) or third (.305) 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 .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 other end of the gauge is for .224 and works the same as the .308. The case mouth should generally gauge to the second (.218) or third (.221) stop and not slide all the way to the handle (.224).

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"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."
— Jason Giglio - Ballistic Tools, Inc.

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