When you fire form a case you want to seat the bullet into lands to eliminate any head space that is there. .
Seating bullets deep does little to prevent misfiring and eliminate headspace. Before you post stuff like this you'd be advised to test it.
-Kill some primers with Kroil. (Don't assume they're dead but most will be)
-make up some dummies, no powder.
-measure them for OAL
-pull the trigger
-re-measure
see??
You're exactly right about the headspace, but seating bullets long doesn't help.
al