Case gauge vs Ammo Checker .... which is which?

Looking at the new to me Lyman Ammo checker. This is the small rifle ammo checker(Lyman also makes a large rifle and a pistol version) I try to explain what the difference is between traditional case/head space gauges and these new types that are cut to minimum SAAMI chamber specs. If you have a gun that has a “tight“ chamber this may be the answer to your chambering problems. Merry Christmas ya’ll!