After the first swap, I quit using no go gages. The Savage system lets you thread the barrel snug on the go gage to get consistent minimum headspace every time. As stated above, put the go gage into the chamber and close the bolt as you are threading the barrel in. When the barrel bumps the go gage, you have a zero headspace condition. When you then tighten the barrel nut the headspace will grow to .002 or .003 which is very desirable.

The no go gage is used to check for too much headspace. The bolt should only partially close on the no go gage.