Thread: Seating depth measurement problems

I've got a 7 SAUM I am developing a 162 a-max load for. I am trying to work out the seating depth to get the projectile just off the lands. I started by seating a projectile in an empty case, hooking it behind the extractor and closing the bolt, progressively seating deeper until I couldn't see rifling marks on the projectile.

I then picked up a Hornady modified case to use with the Hornady LNL OAL gauge tool (thinking it would be more accurate). Using the Hornady tool the distance from the base of the case to the ogive of the bullet is 20thou shorter than using the first method.

I'm not sure why there would be such a big difference, any ideas?