+1 to what RCmodel said.
a difference of .003" is the difference between correct headspace, and dangerously over headspace.
if you want to check the headspace, buy a set of go/no-go gauges. the bolt should close fully on the go gauge. With the no-go gauge, you should only be able to close the bolt about 2/3's of the way. As my instructor told me "you should be able to drop a house on the bolt and it still not close with the no-go gauge inserted"