Pins will always be more accurate than a snap gage. The margin of error using the snap gages is quite large and the worst part of it is that the snap gage only measures 180 degrees of a bore at one time, the pin gage is much more suited to fitting a boolit or a reamer or a pilot to a throat because the pin is a 360 degree finite object. It measures the complete inner circumference of the circle and the complete cylinder at the SAME TIME, something no other gage can do.
The one thing the pin gage can't do is measure ellipse. It can FIND ellipse but not measure it. For this I use a Starrett split ball dial bore gauge that reads in .0001" increments, simply use a master ring to zero the gauge, put the gauge into the throat, read the difference and either add or subtract from the master ring, now you are reading actual diameter in tenths of a thousandth. Simply turn the axis of the probe and watch the dial face, the change in reading is how much of an oval or ellipse is in the throat. Moving the probe longitudinally in the throat will measure taper, or what we call belled/bellmouth.
I have on my bench right now a 357 Maximum cylinder that has some cats eye throats. When I first started using the Starrett dial bore gauge, I was wondering if it was reading true, it was showing .0007" out of rounds on half the throats, and .0003" to .0006" out on the others. It was quite comical really, turn the gauge and watch the dial creeping all over the place and it makes you think well WTF is up wit dat? Then using a pin gage I could look along the sides of the pin when I held it in the light just right, and I could see light on each side of it. The gage pin showed that the dial bore gauge had been telling the truth the whole time. This is another way the pin gage is the superior tool to use for revolver cylinder and autopistol barrel measurements.
There was a 357 cylinder sent to me for reaming, the customer wanted the throats .360" to shoot the same ammo as his levergun liked. He sent a few dummies with .360" boolits seated and crimped for me to use as a gage. I discovered that two of his chambers were reamed so small that even the dummies I had with .358" boolits wouldn't chamber, let alone his dummies with the larger boolits. We spoke on the phone about this and the story is that he sent it back to Ruger, with a note explaining there was something wrong with two of the chambers and they refused to address the issue, sent the gun back untouched.
Since I had both sets of pin gages I discovered that those two chambers had been reamed undersized by a really worn out reamer at the factory, back when they used to gang ream cylinders 3 holes at a time with a Hitachi machine that had 3 cutters. As those cutters wore, they would replace them but they only replaced the worst ones, one at a time, and they left the others until they too wore down too far to use. His cylinder had been made on this same machine, and the one badly worn reamer had cut a pair of chambers that would only accept a .376" pin or something like that, SAAMI calls for .380" behind the chamfer, and I had to go out and BUY a Clymer finishing reamer and redo those two chambers to save that cylinder and get it shooting again. So it left with all .3605" throats and all .380" chambers which he reported back saying the gun had NEVER shot this good in the 20+ years that he owned it.
Pin gages rock! You cannot do this stuff without them.
