The drawing is here
. Notice that under the name of the cartridge it says:
"UNLESS OTHERWISE NOTED
BODY DIA -.008 (0.20)"
Where it gives the neck as 0.253, there is no tolerance noted. That means that, even though the drawing says 0.253", that's only the maximum, and the -0.008" tolerance means it can be as small as 0.245". Note also that this is the number with the bullet in place and not before you've seated the bullet, which will be smaller.
To make seating easier, be sure to have adequate chamfer on the case or get a Lyman M die for your chambering. It puts a small step in the mouth that lets you seat the bullets upright and start in straight.