Does Cast Bullet Hardness Change Over 1 Year

Does Cast Bullet Hardness Change Over 1 Year

Believe it or not, it’s been 12 months since we cast a batch of air-cooled and water-quenched bullets for the series on lead hardness testing.  With a solid year’s worth of results, I’m really excited to see what’s taken place with our alloy, so let’s get started.

Method

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

Air-Cooled

Beginning with the air-cooled bullets, our measurements are:

Bullet 10.063″13.0BHN
Bullet 20.056″16.6BHN
Bullet 30.060″14.3BHN
Bullet 40.058″15.4BHN
Bullet 50.054″17.9BHN

That gives us an average BHN of 15.44.

Water-Quenched

Next, we’ll take a look at the water-quenched bullets.  Measurements here are:

Bullet 10.045″26.0BHN
Bullet 20.045″26.0BHN
Bullet 30.045″26.0BHN
Bullet 40.046″24.8BHN
Bullet 50.046″24.8BHN

That leaves us an average of BHN 25.52.

Results

After twelve months time, the air-cooled bullets have reached an average of 15.44BHN, while the water-quenched variety have stabilized at about 25.52BHN.

Progression over one year

Now that we have an entire year’s worth of data, let’s take a look at the big picture.  If you refer to the graph in the video above, you’ll see it represents bullet hardness relative to the original as-cast value from the day they were first made.

Both sets got progressively harder up to a week, with the quenched test group reaching their highest point at 24% over the as-cast value.

Things got interesting at the three month mark, with the air-cooled bullets peaking at 73% harder than cast.  Meanwhile the quenched set had stabilized at about 22% since the first week.

It’s interesting to see that both sets of bullets softened somewhat at the six month mark, only to harden again at 12.  I’m tempted to call that operator error on the part of the readings, except they seem to be telling a pretty consistent story, and I’ve re-tested both sets just to confirm, with identical results.  It’s also worth noting that all the test bullets were stored indoors at room temperature for the entire duration of this project, so that shouldn’t be a factor.

Summary

Aside from the very obvious fact that cast bullets definitely do age harden, what have we learned from all this data?

If you’re after the hardest projectiles possible, you should be shooting your bullets by about the three month mark.  Air-cooled ammunition should be allowed to age-harden for more than a month, however water-quenched can be used after a week or so.

If you’re the kind of caster who likes to prepare a year’s worth of ammo in a couple marathon sessions, you’re probably more concerned about stability.  That being the case, water-quenched bullets are likely your best choice, with a change of only 2% between a week and three months.  Even if you choose to keep them longer, they’ll only drop 10% relative to the 3 month point.

Lastly, for those who just want to know the final results, air-cooled bullets finished the year at 38% harder than cast, while water-quenched saw an increase of just 12%.

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