I've read this method previously, but never tried it as it seemed so crude and abusive to any fine firearm. The thought of beating on a rod to drive lead into a case and bore just scares me. There's so many things that could easily go wrong.
Considering the price and convenience of Cerrosafe, I much prefer it to pounding on my old single shots.

Marlin, I do not think anyone here meant for you to "SLUG" the Chamber. That is mostly done by making a casting using Cerro Safe or Cerro Bend a Low Temperature material. you Push into the bore just ahead of the Camber a soft Lead bullet to create a plug. Thenwith the Forend removed heat the Barrel at the chamber area until it is JUST uncomfortable to the touch. Then Heat the Cerro Safe with a soft LP torch flame or a hair dryer
till melted. It melts at about 160 Deg F. Use a small Aluminum
or hand made Amuminum foil funnel fill to just shy of Extractor cut and let set until it freezes off. Then using a Cleaning rod from the Muzzle gently put pressure on the Bullet plug and push the chamber cast   onto a Towel or something soft to catch it. The casting usually moves easily as there is significant 
shrinkage on first cooling. Here is where there is some disagreement on dimensions. We had been informed that the material initially shrunk, then with some time grew, then shrank again. This was a cycle that continued for about 24 hours until it settled to the size of the Cavity it was cast in to some very small dimensions. You can then get some really good Measurements from it. HTH Regards, FITZ.

To clear this up. I deleted original post on using a rod to drive lead to slug barrel.  Marlins reply to use Cerrosafe instead...which I agree. Pete

I have a friend who determined the caliber by cramming a candle in the chamber.  His results were questionable.  We are still laughing about it.
James

YEA! Good sense prevails. Regards, FITZ.