from the old forum:
You know it's winter when measuring a throat seems like an interesting thing to do.
We've heard the pros and cons of various measurement techniques debated around the e-campfire. I decided to actually try every method and see how they compared. I marked one hole on an S&W 44 cylinder and measured it 10 times with each method (except I only made 2 cerrosafe casts, and since there were only 6 egg sinkers in the package, I measured each sinker at two different spots for a total of 12 sinker measurements).
Of course, all instruments were calibrated or zeroed beforehand.
Tesa tri-mic this Swiss made 3-point internal micrometer is the Cadillac of hole gages. A set of tri-mics spanning 0.250" - 0.500" costs about $2000. The tri-mic is accurate to 0.0002" and very easy to use. The calibration was checked with a ring gage. Since the tri-mic was the most accurate instrument at my disposal, the tri-mic's average measurement of 0.4335" will be assumed to be correct and all other measurements will be judged by how they compare to the tri-mic.
Expanding ball gages a chinese set of 0.125" - 0.500" ball gages can be found for about $10 and they work pretty well. Some finess is required, and beginners will tend to get undersize readings. The ball gage is expanded to fit the hole, then measured with a digital external micrometer, which was, of course, zeroed beforehand. The ball gage averaged 0.4328", or 0.0007" under the actual diameter.
Dial calipers this particular caliper came from Midway. Equivalent chinese calipers can be found for about $15 (they appear to be made in the same plant regardless of who you buy them from). The chinese calipers tend to be stiff when they are new, and don't hold their zero well, but they improve after they are lubed and broken in. Nevertheless, I always check the zero before using them. Calipers tend to read undersize on small IDs. They are OK for quick, ballpark measurements,but there are better choices when precision is required. The calipers averaged 0.4321" or 0.0014" under the actual diameter.
Egg-shaped fishing sinkers are recommended by Beartooth Bullets. The sinkers were lightly oiled, then pounded through the cylinder with a hammer and a hardwood dowel. The cylinder was resting on a scrap board with a 1/2" hole drilled through the board to pass the slug as it exited. Occasionally the slug would unintentionally get an extra bump after it exited, bumping up the diameter. When that happened, the slug was pushed through the throat a second time. The fishing sinkers averaged 0.4332", only 0.0003" smaller than the actual diameter.
Pure lead bullets These 44 caliber bullets dropped out of the mold about 0.431", so I gave the base a couple of light taps with a hammer to bump up the diameter (you don't need a special bullet for slugging, just use a hammer to bump up an undersize bullet). Then they were lightly oiled and driven through the throats. They averaged 0.4331" or 0.0004" smaller than the actual diameter.
Air-cooled wheel weight bullets These were made and used like the pure lead bullets. The 0.4334" average, only 0.0001" undersize, was more accurate than any of the other affordable methods -- ya gotta love wheelweight. The standard deviation was high due to a single oversize bullet that probably got an extra bump after it had exited.
Cerrosafe At first I attempted to make a casting of the entire length of the cylinder, but that got stuck and had to be boiled out, even though the cylinder had been lightly oiled. So next I made a casting of just the last 3/4". As soon as it had solidified, I quenched it in water, hoping to shrink it. Actually, it was pretty stubborn coming out, but it did come with help from a hammer and hardwood dowel. It averaged 0.4344" after 5 minutes, and 0.4348" after 30 minutes. So then I made another one. This time I drove it out as soon as it solidifed, without quenching in water. It came out with only one light tap. While still warm, it measured 0.4344" and after 30 minutes, it was 0.4351". In other words, the cerrosafe was 0.001+" larger than the true diameter.
Conventional wisdom says that cerrosafe shrinks at first, and then gradually expands, and is supposed to be most accurate after 30 minutes. However, in my experience, cerrosafe starts expanding right away. I have had better luck removing it as soon as it solidifies. If you wait too long, it may expand so much that it can't be removed except by melting. Even though cerrosafe it not especially accurate, it is still useful because it shows you the shape of the throat, which is good to know.
Pin gage I picked up a set of 0.250" - 0.500" pin gages on ebay for about $25 delivered (normally they go for $50 plus shipping). Pin gage sets are either "minus", meaning they are slightly under the nominal diameter, or "plus", meaning they are slightly over the nominal diameter. My set is "plus", and the pins seem to be 0.0001" - 0.0002" over the nominal diameter. A 0.432" pin passed through the throat fairly easily. A 0.433" pin (it actually measured 0.4331") would hang up about 1/8" shy of the exit. Thus the pin gages would have us believe that the throat is larger than 0.432" but smaller than 0.4331".
Now you would think that a 0.4331" pin would pass through a 0.4335" hole, and maybe it would if everything were perfectly smooth, perfectly straight, and perfectly round, but the reality is that some clearance is required to push it through with only finger pressure. That's why, if a cast bullet can be pushed through the throat with only finger pressure, it is too small.