Does Cast Bullet Hardness change over 24 Hours?

Does Cast Bullet Hardness change over 24 Hours?

In my last article, we took a look at some tests comparing air-cooled and water-quenched bullets, and reviewed the data from both.  During the preparation for that post, I took it upon myself to cast enough ‘spare’ bullets of both varieties to perform additional testing in the future.  This article will focus on one of those tests, in this case how much cast bullet hardness changes after 24 hours.

Some folks say quenched bullets will eventually ‘normalize’ back to their original hardness, while others claim they’ll ‘age-harden’ over time.  With differing opinions and no real body of evidence to draw on, I decided 24 hours would be a good place to start.

Method

As with the previous experiment, five bullets each were tested from the air-cooled and quenched lots.  All tests were performed using the Lee Precision lead hardness test kit.

Air-Cooled

Starting with the air-cooled bullets, the measurements were as follows:

Bullet 10.066″11.8BHN
Bullet 20.066″11.8BHN
Bullet 30.066″11.8BHN
Bullet 40.068″11.0BHN
Bullet 50.066″11.8BHN

Water-Quenched

Moving on to the water-quenched bullets, the measurements were as follows:

Bullet 10.044″27.2BHN
Bullet 20.044″27.2BHN
Bullet 30.046″24.8BHN
Bullet 40.044″27.2BHN
Bullet 50.044″27.2BHN

Results

The air-cooled bullets averaged 11.64BHN; that’s only a bit harder than the baseline of 11.16BHN established on the day they were first cast.  The water-quenched bullets were a different story.  With an average hardness of 26.72BHN, they’ve hardened substantially, with an increase of roughly four points on the Brinell hardness number scale.

Summary

Based on these results, it appears that air-cooled bullets change very little after 24 hours, while quenched bullets harden substantially.

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