First, if you want to remove a live primer from a factory round, I don't know of any easy, reliable and safe way to do it. But it would be easy to pull the bullet, dump the powder, decap and re-seat the bullet.
If you are installing the barrel and want to use the factory round as a gauge, the way to do this is to remove the firing pin from the bolt and just carry on with a live round, being mindful of muzzle direction.
But, for setting headspace, no need for the bullet. Pull it.
For checking for throat dimension, that can be set later, using a throating reamer.