Brownells has a .PDF file of instructions for Cerrosafe:
http://www.brownells.com/aspx/NS/General/DisplayPDF.aspx?f=bt002015.pdf. The shrinking is supposed to occur in about 30 minutes, after which the casting is supposed to just fall out, then expand back to size in another 30 minutes. I've never had it just fall out. Re-reading the instructions I believe the problem may have been that I am getting it too hot? The next time I will try a double-boiler to see if that helps?
Meanwhile, I always keep a couple of 1/4" brass rods from Home Depot or Lowe's for various bore-related activities. Smaller sizes can be had from hobby shops, larger from machine tool supply houses. I drop one of these down the muzzle so I can hammer against it to knock a Cerrosafe casting out. That seems to work. A two-pound hammer lets you swing relatively slowly and still have enough inertia to do some work. I also use these rods to push slugs down the barrel when I am firelapping.
If you are only interested in the main chamber dimensions, you can get them from a fired case closely enough for adjusting sizing dies. If you want the groove and throat details, another, perhaps better technique than Cerrosafe comes from LBT's Veral Smith. He says the Cerrosafe castings he's worked with have not always expanded predictably enough. Apparently the shrinking and expanding has to be timed carefully, and is easy to screw up. What Smith does instead is chamber a fired case with a long lead slug in it that is narrower than bore diameter and reaches up will past the throat. He then then drops a rod down the bore and hammers from the muzzle end of the rod until the lead has been cold-forged out into the freebore and throat. What pops out is the right size and stays that way.
In the meantime, I would just wet a patch with spray Pam and push it down the bore for a release agent for Cerrosafe.
Nick