Field gauges are essential tools for ensuring a safe chamber on used rifles. Slightly longer then a No-Go gauge, the Field gauge is used to measure the absolute maximum allowable, safe chamber headspace. If the bolt closes on a No-Go gauge, the Field gauge is used for an additional check. If the bolt then closes on the Field gauge, the rifle is considered unsafe to shoot, you'll have to have the barrel set back and rechambered. Includes a durable, plastic storage case. Available for .308 and .30-06 caliber chamber lengths. Mfg: Clymer SPECS: Steel, precision machined to SAAMI specs. .308 fits .243, .260, 7mm-08, .308, .338 Federal, and .358 Win. .30-06 fits .25-06, .270 Win, .30-06, 8mm-06, .338-06, and .35 Whelen.