When I cast chambers I use a cotton cleaning patch shoved up into the bore far enough that it’s past the throat & rifling lead. This gives the molten metal enough room to fill the bore just ahead of the chamber throat, gives me a good impression of the condition of throat and lands once casting’s pushed out.
I’m also an advocate of oiling the chamber’s surfaces very lightly after thorough cleaning and before making a cast. I know ‘theoretically’ the cast shouldn’t ‘stick’ to a clean, dry chamber but to me a Very Thin film of oil isn’t going to affect a casting’s dimensions beats having to melt a stuck casting out
Keep in mind also that the timing for taking measurements differs from alloy to alloy. Castings shrink as they cool but these alloys then reach a point where they begin to expand again so knowing when a casting replicates a chamber’s dimensions is important for critical measurements. Cerrosafe I think is 30 minutes... so you want to push the casting out with a cleaning rod well before this time limit or you may have to melt it out.
