I always drop the sprues back in the pot. My bullets get heavier as the mold heats up.  When the mold comes up to temp and I don't get distracted, have had batches of bullets weigh out to about six tenths variation in weight.
Now days seldom weigh bullets. 

           Joe.

would small temp variations in the molten alloy = have an effect on the molten lead density?   I wonder if the mold block temps /expansion might have an effect on the way the cavity halves fit and perhaps the cavity dimensions.

DW expansion and contraction I would think would be based on the rate of rise and or drop in block temperature and well beyond my abilities to calculate. I do think for our use being based on the mass involved it would be so miniscul that it would be almost unmeasurable? When casting the temp. is always on the rise and or stable as one moves along at a rapid pace so where would there be an extreme enough drop in tempreture during this process take place. I have only found it to take place by putting the sprues back in the pot while casting and that variable is easily eliminated.

JLouis

My records of visually inspected cast bullet weights of 26,948 bullets, from 58.5 gr. to 445.3 gr., has an average standard deviation of .155 grains. 
Standard deviation varied from .062 to .309 grains, variation had much to do with casting technique-my fault and generally identified as such.
The standard deviation doesn't seem to vary MUCH with average weight. 2 cavity molds are marked to identify bullet to cavity, bullets are sorted by cavity, bullet weight varies, consistently, by cavity. 
Average lot size is 110 bullets, a lot = from a cavity at one casting session.
John Alexander at the CBA doesn't weigh or pay a lot of attention to visual defects, and shoots very well. 
Weighing bullets after visual inspection at 4X lets me identify the very few visually acceptable but weight-outlier bullets. 
Repeated testing has failed to show that accuracy varies with bullet variation within +/-3s. Bullets weighing XXX.Y grains shoot no better than bullets weighing XXX.0 to XXX.9 grains.
You will find that good bullets shot in the order cast do NOT shoot more accurately than good bullets segregated by lot.
You will find that bullet weights varying +/- 3s, where 3s = .5 grains when X bar ~ 200 gr do NOT shoot less accurately than bullet weights with the same tenths digit.
You WILL find that weighing bullets and calculating s DOES help you to control casting variation by showing you WHAT causes variation.
I have an excel workbook that automatically calculates s and x bar and records data, available to all. 

Quote:

The bell curve can be tightened up ...

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100 bullets (9.5x47R) ladle cast with Accurate mold
Heated the pot up to 780 degrees - (the keys to small bell curve = cast with a 5 second pour - sprue puddle frost in 7 seconds) and the results:
* 04 bullets = 209.8 - 209.9 
* 96 bullets = 210.4 - 210.8
Cast with a constant: melt temperature, 5 second pour time, frost time (5 - 10 seconds depending on weight) and with a uniform rhythm for small bell curve weights

John -  cool!  Describe in a bit more detail about 'constant pour time'.  Pouring directly into the hole or glancing off the funnel in the sprue plate or pressed directly to it?
Thanks!

Which scale are you using?

JLouis wrote on Nov 24^th, 2013 at 12:15am:


Agree.  Might also there be an effect of the volume of the alloy expanding with temperature AND the dimensions of the cavity varying with the temperature of the mould.  Which has the greater affect?

With my most recent casting of 340gr 38cal bullets and 400gr 40cal bullets was able to hold +/-0.2gr for 98% of the bullets. I give credit to 1.) casting consistency - doing it the same every time (just like shooting ), and, 2.) using a PID control and and Type K TC to run the pot. The PID controller is one of the 'fuzzy logic' learning type of controllers (~$35 with an SSA - solid-state relay on eBay). Bought the TC from Omega Engineering and it was about the same w/shipping. For $70 have a modern feedback control system and do not have to pay attention to pot level. Will generally cast 100 bullets and then take a break and feed the pot. 

Have learned recently that there is a big difference between molds. The BACo molds, which are a large block, heat quickly and maintain a stable temp quite well. The Brooks mold is a smaller block and takes 15-20 bullets to stabilize, but is great after that. The Saeco 640 40cal mold is easiest/fastest to use. Rereading the notes on each mold before starting to cast makes things go faster. YMMV.