Cutting Edge Bullets
They give you some leeway in their instructions and there is also a desired fit.
The rear of the STB would be seated against the case mouth.
When chambered, the front of the STB would be about .025" from contacting the freebore. The DB would be resting in the freebore and would be more than .025" from contacting the throat. The borerider will be resting in the bore.
When fired the DB would not make contact with the throat until after the STB has fully entered the freebore.