Some neck sizing plugs prepare the case for .451" bullets and you may be using .452"'s. A good micrometer is a wise investment and will last a lifetime if you care for it.
If you back the seater up to clear the crimp and use a separate crimping operation you will get more uniform ammo. You can use the same die set closer for the crimp and the bullet seater to just clear. Last, you should dust the inside of the case mouth with motor mica on a brush. Midway sells the whole kit to do this for very little cash outlay.