OAL gauge

[ar15barrels]

That is .0015 swing from head to O-give and a .0125 swing in COAL.

The difference in swings of the two different measurement methods is because the ogives are far more consistent than the bullet tips.
You can also see this if you took 10 bullets and measured the ogive length of JUST the bullets and then measured the OAL of just the bullets.

[JDay]

It's probably one of those tools that uses calipers.
Calipers don't work well for people not super familiar with using calipers.

Having one of these helps.

http://www.sinclairintl.com/reloading-equipment/measuring-tools/bullet-comparators/hornady-lock-n-load-anvil-case-prod41213.aspx