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I knew that the alloy temperature affected bullet weight but had never determined how much. Normally I went with the temp that resulted in fully filled out bullets using a specific alloy. Since I planned to cast sufficient bullets for 3 silhouette matches I figured I’d run a simple experiment. So I just finished casting & weighing 3 batches (60 bullets/batch) of .40cal 400gr bullets using the BACO 409400M5 (old 409400M4) mould and the RCBS Easy Melt PID controlled furnace.
The 1st batch was cast at 765 degrees & weighed 400.0grs +/- 0.6grs. The alloy temp. was increased to 770 degrees for the 2nd batch which weighed 401.0grs +/- 0.3grs. So then I lowered the alloy temp. back to 765 degrees for the 3rd batch with dropped back down to the same weight as the 1st batch (in this case the weight was 400.0grs +/- 0.5grs). So now I know that 5 degrees will result in a change of approximately 1gr.
BTW, the alloy was verified as 15.7:1 by measuring the specific gravity using a laboratory grade high precision scale.
NRA Life (Benefactor) Member, TSRA Life Member, NSSF (National Shooting Sports Foundation) Member, Author/Publisher of the Browning BPCR book.
|one of us|
I tend to cast at about 750F for my Sharps too. With 20:1 alloy my 480 grain Saeco mold puts them out at about 490 grains. I will end up getting some other weights but they are generally within 3 grains so that's less than 1% variance which suits me. It seems there is always one or two bullets in my batches of 100-150 that are somehow out of that 3 grain spread. I do store them by 1 grain differences though.
In loading them and shooting the cast bullets I have found little to no difference in the point of impact of the slightly different weight bullets. Virtually all of the accuracy of the rifle and cartridge is there. I attribute an inch between a shot, or a half inch between shots, as both my shooting and probably the sight picture too.
I just load and shoot mine at informal targets and I use it for hunting. For long range shooting the effect of the weight differences could be and would be larger.
Thanks for your work Mac and for sharing it too.
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