Shortly after mentioning in another thread that I have the correct expansion factors when using CerroSafe, I received requests for the factors.  So, I’m posting the info here for others that may be interested.  The following is from a much longer article I wrote in 2020 on making chamber casts.  For the complete article including other chamber cast methods, go to (You need to Login or Register to view media files and links)

Wayne

Adjusting Data Taken From “Aged” Casts
Most shooters make a chamber cast, take some measurements and never use the casts again for various reasons.  The most obvious reason is, once accurate measurements are made and documented at the correct time, there’s little or no need to revisit the cast.  Another reason is they are aware that the cast will expand over time but are unsure how to determine the original dimensions from the aged and expanded cast.  For several reasons I’ve had the need to use old CerroSafe casts to verify original measurements and/or take dimensional data from different locations than originally measured.  In doing so I came to the conclusion that the published expansion data are flawed as is the method to use it.  The bottom line is the expansion data that has been published to date is very old (dating back to 1934) and I experimentally determined it to be incorrect.

Correctly Adjusting Data taken from “Aged” Casts
Based on the experimental data, the expansion factors listed in the table below are what I now use.  The factors are in inches per inch.  Therefore, to calculate original dimension (X) from aged dimension (Y) use the equation X = Y ÷ (1 + Factor).  For a close approximation use X = Y (1 - Factor).  For example, using 0.461” for the groove diameter of a 200 hour or older age 45 caliber cast will result in a value of 0.4587”.  For this example, the equation is X = 0.461” ÷ (1 + 0.0051).

So, if you failed to measure the cast at the recommended 1.5hrs after making it or you’ve been holding on to old CerroSafe chamber casts, pull them out, take some careful measurements and apply the formula and expansion factors listed above.  The results should approximate the chamber, throat and bore dimension of your firearms.

Does a sulfur cast change dimensions?

Great info thank you for sharing