Here's some instructions I've saved for doing one with "flowers of sulfer" or Cerrosafe either one. Feed stores have the sulfer too and it's much cheaper than Cerrosafe, so I'm told.
Hope this helps some.
I've never used Cerrosafe. Sulfur is a yellow mineral very common in any home improvement store, garden supply or drug store. It's about 10% of the price of Cerrosafe. "Flowers" of sulfur just means it's more pure.
Clean the chamber very well and then swab with a very light coat of light oil. WD40 works fine for this. Stick a patch about half inch ahead of the throat. Now melt some flowers of sulfur over low heat ( you DONT want it to catch fire...it STINKS!).
Heat the barrel in the chamber area with a propane torch (Yes, the action better be out of the stock)
until its just too hot to hold on to. Now carefully pour the melted sulfur into the chamber all the way to the top. you'll see a funnel appear in the center. that's normal. Wait about five minutes and gently knock the cast out and be sure to catch it. The dimensions will be about .0001 smaller than actual...that's plenty close enough. WRITE DOWN THE MEASURMENTS!!!!! You'll need it later and the cast will change and become smaller with age. Use the measurements you get when its fresh.
You can do the same with your dies if you want to get really curious.
A loaded round MUST measure at LEAST .002 SMALLER than the neck of the chamber. A fired round should not be more than .005 bigger than a loaded round. A RESIZED case should be no more than .002 smaller than a loaded round. You VERY seldom find commercial chambers, and dies and neck thickness’ that come close to this.
I would ad to Steve's directions, that I place a light coat of oil in the barrel & chamber before pouring. And I use a borescope to set the cleaning patches only 3/4 to 1" into the rifling.
I have cast ALL my match chambers. But I usually do it when the barrel is off the rifle. It's a lot easier that way.
Be sure to clean the bolt raceways and lug engagement areas before use. I have had a little piece cause me BIG headaches. It didn't hurt anything. I just couldn't figure out whay it was so tight to cam over.
I push two (one at a time) cleaning patches into the barrel so they end up two inches ahead of the throat, to form a dam. I have ground a dewey style jag flat (blunt), so the patch will stay ahead of the jag, and I can remove the rod and jag without pulling out the patch.
I then clamp the rifle muzzle down in a padded vise. You can pour from the rear of the action (bolt removed) or you can pour through the ejection port of the action.
I then make a small funnel from a piece of aluminum foil. I form the narrow part of the funnel around a pencil, and remove the pencil when finished. I make the funnel so that the large end will rest on the action, and the bottom of the funnel to just go inside the chamber. This allows me to pour the liquid cerrosafe into the aluminum funnel and have it trickle into the chamber. When I can see the cerrosafe filling to within 3/8" of the top of the chamber, I quit pouring the cerrosafe, and let it cool according to the package directions.
IF YOU POUR TOO MUCH, AND IT GOES WHERE IT SHOULD NOT, DON'T WORRY. YOU CAN EASILY REHEAT THE CERROSAFE WITH A BUTANE TOURCH AND JUST POUR IT OUT, AND START AGAIN. THE CERROSAFE WILL NOT ADHERE TO ANY METAL!
I then carefully inset the dewey rod (with the blunt jag) into the muzzle end of the barrel, and give it several good "rapps" with the heal of my hand. Each "rapp" will move the casting rearward a little bit, and you can feel how much it moves depending on how hard you "rapp" the cleaning rod. Push the casting up into the action until you can easily grasp it with your fingers.
Measure according to the directions "cooling period" of the cerrosafe. Have fun!