cerrosafe cast Works best on single shots. If you use it on a bolt gun, be careful not to let it spill inside the receiver. I haven't had good luck using cerrosafe on lever guns -- it's prone to spill into the nooks and crannies of the action, and it's prone to get stuck in straight-wall chambers.
Plug the barrel about a 1/2" in front of the chamber. A very tight fitting cloth patch will usually work.
Some people like to oil the chamber, to facilitate extracting the casting. I usually don't bother, but then, I've had a few castings get stuck.
I use a small stainless sauce pot that I bought at a 2nd hand store for 50 cents to melt the cerrosafe. An RCBS ladle for pouring (clean the ladle to prevent cross contamination with your bullet alloys). Heat the cerrosafe just enough to melt it -- avoid overheating the cerrosafe, because the cadmium fumes may kill you.
To pour into a bolt gun, you'll need some kind of funnel. I've use a dogleg shaped piece of 3/8" aluminum tubing. One person found a tea kettle that worked well for both melting and pouring the cerrosafe. You can usually pour into single shots directly from the ladle.
Once you have successfully poured, the trick is to remove the casting at exactly the right time. Because of its low melting point, the cerrosafe takes a long time to solidify. Maybe 3 minutes, or maybe 10 minutes, depending on the temperature of the cerrosafe and so forth. It's hard to predict.
If you try to remove the casting before it solidifies, liquid cerrosafe may spill out and get on the floor and the walls and the furniture, etc.. I learned that the hard way.
On the other hand, the longer you wait, the more likely the casting will expand and get stuck.
Contrary to what you may have heard, cerrosafe does not necessarily shrink for the first 30 minutes. In my experience, it expands almost right away.
I've had the best luck removing the casting as soon as it solidifies, usually after 5 - 10 minutes. It should only take a light tap.
If it doesn't want to come out, don't beat on it. Admit defeat and use either boiling water or a hair dryer to melt the casting and start over.
Assuming the casting comes out, I dunk it in water to cool it, then measure it right away. Don't worry about casting imperfections -- just avoid measuring on the imperfections.
Over time, the casting will expand to be 0.001" - 0.002" larger than the true diameter.
Cerrosafe downside: it sometimes gets stuck, and accuracy is not what it is cracked up to be.
Cerrosafe upside: it shows the shape of chamber better than any other method.
sulfer cast. Procedure same as cerrosafe except using sulfer that you buy from a pharmacy. Roy Dunlap's gunsmithing book has a detailed description. I've never tried it but have heard nothing bad.
upset slug Get a dummy case with a spent primer. It is sometimes helpful for this dummy case to have a short neck, because that will result in an impression of the neck in addition to the throat.
Pour pure lead into the dummy case, 'bout halfway up the neck. Now cast a few bullets out of pure lead -- either the same caliber or one size smaller. Chamber the dummy with the pure lead bullet (or if the bullet is a smaller caliber, just drop it down the barrel). You'll need a metal rod, either steel or brass. Put a few peices of masking tape on the rod here and there to minimize contact with the barrel. Insert the rod down the muzzle and smack the rod several times with a hammer until the pure lead slug has upset to fill the throat. See the thread on the 38-55 for an example of the finished product.
With either of these methods, you often have to redo it two or three times before it comes out just right. The idea is to make the impression just long enough to capture the entire throat and leade -- but if it is too long, then it may be hard to extract.
Things to look for, once you have a chamber impression.
1) neck clearance. About 0.0025" clearance is ideal for everyday shooting. Some chamber necks like the 38-55 are so tight that they will not chamber a cartridge loaded with the correct cast bullet diameter. Other chamber necks like the 30-06 have excessive clearance that can contribute to gas cutting and misalignment.
2) freebore length. If the freebore is very short, then some cast bullet designs will not chamber. If the freebore is very long, some cast bullet designs will not be able to reach the rifling.
3) freebore diameter. Generally, the bottom half of the bullet should be sized at least as big as the freebore diameter -- but some freebores are so grossly oversize that a good CB fit is not feasible.
4) leade angle. CBA benchrest guns have very gentle leade angles, typically 1/2° - 2°. Some SAAMI chambers like the 32-20, 348, and 45-70 have very steep leade angles, which tend to be rough on cast bullets.
5) if your impression captures the beginning of the rifling, then you can measure the groove diameter and bore diameter (in the case of a modern custom barrel, we can safely assume that the diameters are close to spec). A blade micrometer is required to measure the bore on the impression, and it's a tricky measurement. The groove and bore dimensions will often be oversize just in front of the chamber, due to heat and erosion.
Measuring your bore (not groove) diameter
As mentioned earlier, most chamber impressions also give an impression of the barrel just in front of the chamber.
I prefer to measure the bore near the muzzle using an expanding ball gage (you can get expanding ball gage sets for $10 - $15). This is pretty easy to do and then you measure the gage with a micrometer. Just bear in mind that the bore may be eroded just in front of the chamber. In fact, the bore will continue to erode as more shots are fired, so a bore riding nose that fits perfectly today may not fit perfectly next year -- just one of many problems with bore riding noses, but that is another story.
Pin gages are often used to measure bores, however, I haven't found pin gage measurements to be very accurate, because clearance is required to slip a pin gage through the bore. Nonetheless, the pin gage method is popular because it is very easy to do. Just bear in mind that the true bore dimension is likely larger than indicated by a pin gage test.
Measuring the groove diameter
At the risk of shattering popular cast bullet wisdom, I'll state that most modern barrel grooves are close to spec and I don't usually bother measuring them unless I have reason to suspect that something is wrong.
Old guns are another story.
Modern guns chambered for old black powder cartridges should also be suspect. For example, 38-55 groove diameters may range from 0.375" - 0.381".
A chamber impression will often capture groove dimensions, with the usual caveat that the groove dimension in front of the chamber may be enlarged by heat and erosion.
If you slug your barrel, be aware that sometimes the slug may abrade as it travels down the barrel, resulting in an undersize measurement, sometimes way undersize.
Also, some barrels have restrictions (i.e., caused by punching the lettering on the outer surface of the barrel) that will cause a slug to come out undersize.
If the gun has 3 or 6 grooves, the grooves can be measured very accurately from the muzzle with a tri-mike. However, tri-mikes are very expensive, so the average guy is not going to have tri-mikes laying around. I do, and I prefer to use tri-mikes to measure barrels.
If the barrel has 5 grooves, you may or may not be able to measure the slug with a conventional mic. Sometimes you can cheat and measure a 5-groove slug if you position it just right, or if you twirl it as you close the mike. It sounds lame, but it can give decent measurements with practice. Otherwise, you need either a special mic or a special fixture to measure 5-groove slugs.
Imported mics can be purchased for $10 or less so there is no excuse not to have one, along with a $20 imported caliper.