I understand what you are saying TiredIron and you are correct. Most times, I have found the calculated (from measuring with a regular ruler) caliber to be helpful and close. In working with OLD hand-molded and oxidized bullets, the measurement you get varies according to the condition of the bullet and the accuracy of the reader/instrument. So when I said that caliber = diameter in decimal inches, I mean that the number you get will correspond closely to a caliber and can be further IDed when the calculated decimal diameter is compared with the value in a table of known period calibers. The decimal diameter number is a good number to start with and it is pretty close to the old calibers. It is almost impossible to determine the exact caliber from a old shot bullet but I have found that it is usually very close. Close enough for government work as they say. Especially if you don't have access to calipers or a micrometer. Some of the calibers are so far out of the range of others that +/_ .02 or so won't lead you in the wrong direction. In the case of this bullet, 1/2" in diameter - about a .50 cal - is most likely NOT a pistol bullet.
With modern copper-jacketed bullets, it is much easier to get a measurement pretty close to the what the bullet was when it was made. Looking up that value in a chart will be pretty much right on.
So my "field" method is not as accurate but is close and using the decimal diameter as the caliber gets me to the right chart or webpage. Your methods are much more accurate and modern and if the measurements are precise, will tell the caliber of bullets that vary significantly from their decimal diameters.
Thanks for the comments to keep all the information as useful as possible.
Daryl
Thats a lot of leeway....so I'm assuming its "not"....a 50 cal and closer to something in the 44cal to 45cal range by his description. As usual..... too much accurate information is better than little to none. A projectile in this good of condition is usually a piece of cake to identify "what it is".....not so much when it was made. Weight is everything.
Actually I think Tnet was having a little hiccup when you were trying to post because I was having problems at the same time also. Still funny though. 
