LaPala:
I didn't do a statistical analysis to find the average weights, but I checked a couple of times and there was a small variation of +- .1 grams, probably due to the sprue nubs. The bismuth was from store bought "bismuth sinkers", and I only have about 3oz. I had to cast a bismuth sinker, weigh it, then remelt and recast it, the weight remained practically the same each time (I reused the solder as well). I used an Ohaus triple beam balance (.1g graduations) to weigh all the sinkers.
I calculated the volume for the 1 ? oz sinker to be 3.8 cubic centimeters. I measured the volume indirectly using Archimedes' principle. I hung 3 sinkers by a thread from the balance, weighed them "in air" then "in water". The difference in weight was 11.4 g. 11.4/3=3.8. 3.8grams of water has the volume of approximately 3.8cc
For more info try these sites:
http://www.matweb.com/index.asp?ckck=1
http://www.magmaengr.com/books.html
http://www.lasc.us/CastBulletNotes.htm
http://www.nwtskirmisher.com/useful-casting.shtml