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 tnsion on the calipers can give distorted readings. Improper neck tension can make preojectile insertion more difficult, or even cause a safety issue if the neck is too loose. This gage is for 224 diameter and 308/310 diameter brass. It will do cartridges such as .223 Remington, 5.56mm NATO, 7.62mm NATO, .308 Win, .30-06, 7.62x39mm, and many more.
Sinclair, Issue:5B, Page:031
SPECS: Steps are .218, .221 and .224 on the 22 side, .302, .305 and .310 on the 30 caliber side.