Wierd bullet weights

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Sudbury, Ont.
About 15 years back, I bought about 150 pounds of linotype metal, still in type form, from a local scrap yard. I rendered it all down into ingots and have used very little over the years. Last year, I cast several hundred of the RCBS 30-155-SIL bullet and threw any with defects back to be remelted. This was done over several casting sessions.
Last winter, I picked up a CIL 950T, heavy barrelled single shot .308 target rifle that I fitted with iron sights. My first 5 shot group shot during the winter was under 1/2 inch at 50 yards.
Now, over the past month and a half, I've been getting poor results. My best ten shot group was just over three inches at 100 yards. I've just started weighing these bullets and most were in the 151 to 154 grain range. When I started on the second jar of bullets, they were immediately about three grains higher. Here's the distribution.



There is a real drop at the 155 grain mark which is right in the middle of the pack, where one would expect the largest amount to be. Well, I know that it was all printer's metal when I started, but it appears that some of the lighter elements like tin and antimony has been burned of part of the mix, because some bullets are noticeably heavier than others. It underscores the value of weighing bullets. As mentioned, all the bullets were first examined for visible flaws. I'll start shooting the heaviest bullets first, and work my way down, and see how important consistency is.
 
That is very odd that the two largest groups are 3 grains apart. In my experience the largest groups are usually only a grain apart; maybe 3 largest groups all consecutive weights.
 
There can be quite a difference in weight if the temperature of the mold/melt is not consistent. Check for lead smear under the sprue plate. That can increase the weight too.
 
This could also be caused by voids in the bullets or a inconsistent alloy. Generally I will try to melt a large batch and then pour into ingots to be used later with a batch # stamped to avoid differences in alloy content.
 
Linotype from different sources can vary somewhat. The variance may only be a a percent or two in the ration of lead/tin/antimony but that would be enough to make a grain or two difference in bullet weight. Throw in some variance in casting temperature and fill out, and suddenly 3-4 grains difference is not hard to imagine. Can't see the bullets well enough to comment on fill out, but that is what usually causes the most variance.
 
Good thought TJ. The RCBS moulds are all double cavity, but since all the heavy bullets came from the second jar, it's unlikely that all the heavy bullets over three or four casting sessions would end up in one jar, and one jar only.
All the bullets were inspected and unfilled ones were tossed back in the melting pot.
 
Chalk me up as another guy that figures that you changed up your alloy at some point along the way.

Esp. if you have the results of 3 or 4 sessions, all amalgamated together, and you topped up your pot from your loose letters and spacers supply.
Better to melt the whole lot of pieces as a bunch, then cast clean ingots of that, and work from there until it is gone. You may not match it exactly the next time, but for the time it lasts, all the alloy should be the same.
The possibility of the letters and spacers being of adulterated alloy or slightly different alloys, is there too. If you look in to how a print machine works, the print would get recycled until it caused casting problems, in some outfits, then virgin metal ingots would be added until the letters cast cleanly again. So I have read, anyways. Side note. Neat machines, the typeset machines that made those letters!

Another thing that can do you a bit too, which may not be THE answer, but may be one of them, is that your pot was not very consistent temperature, which can and will change fill-out and bullet weights.

But I would consider looking for a friend with a hardness tester and test a few of each batch. Or simply back up and melt the whole lot, and do a run with the scale on hand to cull those that are outside your preference of range.

Cheers
Trev
 
I don't think anyone snuck in and did anything. :) Did you measure as-cast diameters? If the casting temperature wasn't the same or swung during casting then the bullets can be slightly different diameter. Hotter alloy will give a smaller diameter esp when there's a lot of animony and tin in it. Different diameter gives you different weights.

If you had some decent closeup pics I could tell you more.
 
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