Does Cast Bullet Hardness Change over 6 Months

Does Cast Bullet Hardness Change over 6 Months?

It’s now been six months since we made a batch of air-cooled and water-quenched for our series on lead hardness testing. Let’s see where things are at.

Method

As with the previous experiments, five bullets from each group will be tested using the Lee Precision Lead Hardness test kit.

Starting with the air-cooled bullets, we’ve got measurements of:

Bullet 10.063″13.0BHN
Bullet 20.057″16.0BHN
Bullet 30.056″16.6BHN
Bullet 40.066″11.8BHN
Bullet 50.064″12.5BHN

For an average BHN of 13.98. While that’s still harder than the 11.16 they were cast at, it’s actually quite a bit softer than the 19.3 they measured in at just three months ago.

Let’s check the water-quenched bullets next. Our readings here are:

Bullet 10.046″24.8BHN
Bullet 20.046″24.8BHN
Bullet 30.048″22.7BHN
Bullet 40.048″22.7BHN
Bullet 50.044″27.2BHN

With an average BHN of 24.44. Again, like the air-cooled bullets that’s quite a drop from the 27.76 these bullets showed just three months ago, although still harder than the 22.76 they were originally cast at.

Results

After six months time, the air-cooled bullets have reached an average of 13.98BHN, while the water-quenched variety have stabilized at about 24.44BHN.

Summary

So does cast bullet hardness change over six months?  Absolutely, and probably not how you’d think.

While there’s always the possibility of anomalous test results, it would appear that air-cooled bullets begin to soften substantially as they approach six months of age.  In the case of our test bullets, they’ve dropped roughly 28%, leaving them at an overall hardness increase of just 25% over their original cast value.

The water-quenched bullets seem to be telling a similar, if less-drastic story.  Like the air-cooled, they’ve softened by about 12% since we tested them three months ago.  That’s brought them down to 7% harder than they were originally cast.

I have to admit I’m pretty surprised by this; I was expecting them to either continue hardening a bit more, or maybe just remain static.  It’ll be very interesting to see if this downward trend continues to the one-year mark.

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