Heat Treating Cast Bullets For Better Performance

WARNING!

This article addresses issues that deal with reloading firearm cartridges. The information presented is a result of our experimentation and experience. We offer no guarantee or warranty of any kind on the information presented and you should proceed with caution if you choose to try the techniques or products that we present. Reloading is not an exact science; we have no control over what you do or what you use. Therefore, you assume any and all risk involved.

Cast bullets give us new freedom when we reload. Moulds are available in an astounding array of weights and styles. If that is not enough, and you have an idea for the next great mould design, then the good folks at Lee Precision would be happy to make the mould for you. All it takes is time and money. Articles here at surplusrifle.com have covered well the tools, supplies and basic procedures so one can produce clean, filled out bullets. See �Making the Surplus: Reclaiming Wheel Weights�. In that article, Jamie briefly mentioned two methods to heat-treat cast alloy bullets. In this article, we will look in depth at those procedures for improving the strength and hardness of our cast bullets.

Cast bullets can be made from various alloys. Linotype, because of its casting and strength properties is the preferred alloy. However, as �hot type� printing disappears, and photo offset printing continues to grow, linotype gets harder, and, increasingly more expensive to come by. We need a cheaper, easily obtainable alloy. There is such an alloy, and it�s no further away then the neighborhood tire store.

New York, 1886�

Ottmar Mergenthaler (1854-1899), a German born knife, tool and clock watchmaker, invented the Linotype machine. Essentially, before the Linotype machine was made, printing was done by individually placing single letters to form words, words had to be placed into sentences, etc. This machine was able to cast an entire �lines o� type� (Linotype) into a molten, fast-cooling alloy which was then used in the printing process. It was the most common way of printing until the use of offset printing came into place in the 1950�s where photographic techniques made use of full size printing plates.

As world progress moves forward, let�s be very thankful the lowly wheel weight is one item industry has not figured a way to make from plastic! Wheel weights are still made from lead alloy, and will be for at least the foreseeable future. Most tire stores will either give you a bucket full of used weights or let them go very cheaply.

Lead and lead alloy hardness is rated on a scale known as the Brinell scale. Just like steel alloys are rated on the Rockwell scale. To begin with, let�s look at several alloys and their ranking.

Pure lead, as used for muzzleloaders and black powder cartridge arms, has a Brinell hardness of about 5; wheel weights have a brinell hardness of about 8 or 9. Linotype has a brinell hardness of about 22, making it an excellent alloy of bullet casting.

The reason we say �about 8 or 9� is that wheel weight alloy can vary, depending on the producer and when the wheel weight was made. Very old wheel weights were a bit harder than the newer ones. The same applies to so called �pure� lead. Industry has standards, however a bit of leeway is usually present.

The higher pressures and powder burning curves associated with smokeless rifle loads usually do better with a bullet of a higher Brinell number. Harder, stronger bullets are less prone to deformation and gas cutting (or having the gas escape from the side of a bullet) then very soft ones. So, what we want is an alloy with the strength and hardness of linotype, but the price and availability of wheel weights. Provided we are willing to take one extra step or two in our procedures, wheel weight alloy will serve just as well as the expensive and elusive linotype!

Pure Metal vs. Alloy

First off, getting a 100% pure metal is very unlikely. There is always some sort of impurities in the metal. Never mind the cost of a certified pure metal source. An alloy, on the other hand, is a combination of metals that are put together in known amounts to enhance desired characteristics of the final metal. Like antimony is added to lead to give it more hardness, tin is added to give the metal more fluidity to be able to fill a mould better. Stainless steel is an alloy of iron, nickel and chromium, giving the iron hardness and corrosion resistance. Gold jewelry usually contains an alloy of gold and nickel (the nickel making the softer gold harder).

Just as steel can be hardened or softened by using various techniques, bullet casters can harden wheel weight alloy to perform like linotype in our rifles and magnum handguns. These techniques involving quickly �quenching� or cooling a hot bullet in water.

The first technique involves using an oven to heat the cast bullets to just below melting point, and maintaining that heat for a period of 30 minutes to 1 hour, then quenching the bullets in water. The hardening process starts as soon as the bullets are immersed in the water. After being removed from the water, the hardening process continues for a period of two days, by which time the bullets have reached their maximum hardness. Typically, people use their household oven to heat the bullets. The second method involves getting the bullets, while still hot, right from the mould into a container of water. Of course, there is a lot more to both methods then the brief description I have just given. In this article we will examine both methods of heat treatment in detail, and see what each has to recommend.

But, before we get to the actual �nuts & bolts� of heat treatment, let�s have a short discussion on chemistry (I promise it will be short). If you hear someone laughing in the background, it is my old college professor; He said we would need this stuff one day!

When you heat an alloy or pure metal, the molecules in the metal become mobile, the more the metal is heated, the more the molecules can move around. Once the metal is allowed to cool, the molecules in the metal will reform into a solid mass and settle down into distinct crystalline structures. Hardness is directly linked to the crystalline construction of the metal and how the molecules of different metals in the allow come together.

Wheel weights, are composed roughly of 95.5% lead, 4% antimony, and 0.5 % tin. Tin is added to increase the �filling out� of the mould. That is, it allows the molten lead alloy to flow smoothly and fill the mould completely by reducing the surface tension of the lead. Antimony is used to give some molecular level hardness to the lead. Too much antimony in the lead alloy and the metal can actually become brittle enough to break on dropping. To get the best results, both tin and antimony must be present.

When the lead alloy is in a melted state, the small amounts of tin and antimony are dissolved into the lead. If this alloy is cast into a bullet, and then allowed to cool slowly, the tin and antimony will have time to precipitate out separately. If, however, the metal is quenched, the antimony and tin is trapped within the lead crystal structure, making it harder.

Even after the quenching, the bullet will �age� harden at room temperature. That is, over a period of time, like days, the metal will continue to get harder on the Brinell hardness scale. In industrial situations, metal can be age hardened faster by keeping it at a heated temperature (much lower than the quenching temperature) for a period of time.

A word of caution:

Lead can be introduced into the body in two ways, either through ingestion, or inhalation. Wearing gloves and washing hands takes care of the former, to address the later we need to do a bit more. One can either cast out doors, or have a positive ventilation system. I chose to do the later. I acquired an old exhaust fan from a gas furnace, and mounted it right above my casting furnace. It exhausts directly out the window. Another note. When casting, one part of the technique involves removing the dross, or �dirt� and undesired impurities from the molten alloy. This dross contains lead oxide, which is particularity toxic inhalation hazard. Dispose of this promptly; do not allow it to accumulate in your casting area.

Fumes from flux being drawn up by exhaust fan.

Some necessary casting equipment: wool glove, folded towel, plastic hammer, moulds, container of fluxing material, matches for smoking Lee moulds, old mess kit tray for sprue collecting, and an ingot mould.

The quench-from-the-mould heat treatment process is very simple. I drop bullets from the mould unto a folded towel while wearing a 100% wool GI glove liner. I then grab the bullets and drop them into a water filled plastic tray I got from the Dollar Store. The tray has a folded towel in the bottom. The water in the tray does not need to be ice cold, just insure the water is 3 to 4 inches deep. The first few bullets that are cast will not be the best nor ideally hot enough for consistent heat-treating. To test this, I drop the bullets into one corner of the tray and listen for a sizzle sound. Once the sizzle sounds the same for bullets as I cast, then I can assume that I am getting the same hardness treatment on the bullets. Only those bullets, the ones that sizzle, are used for shooting. The few bullets in the �Non-Sizzle� corner will be remelted later. After a casting session, all the sizzle bullets are spread out on a towel to dry. They will reach maximum hardness in two days. This is the method of heat treatment I always use. It is simple, quick, effective and cheap.

SAFETY NOTE

NEVER TOSS A WET BULLET BACK INTO THE MOLTEN POT OF ALLOY. THE DAMPNESS WILL CAUSE A DANGEROUS EXPLOSION OF MOLTEN LEAD TO OCCUR.

Plastic quench tray with cloth pad and water

The oven method is a bit more involved, yet some swear by it. Simply drop bullets from the mould unto a folded towel and let them cool. Next, preheat an oven to 450 degrees and place a metal tray (which will NOT be used for food) with bullets in the oven and let them heat for one hour. After that time, get them into a container of water quickly after removal from the oven. They too will require 2 days to reach full hardness.

An interesting note is that the temperature of the water used in the oven heating quenching method DOES have a positive effect on the hardness of the bullet, unlike the in the mould-to-water method. For the oven method, use water that is as cold as possible to obtain the best results, however, if you are doing batches, you will need to replenish the cold water as you change batches to obtain consistent results.

Homemade tray with bullets ready to go into oven

Bullets being quenched in sink after oven heating

In the past there was no affordable, ready-made tool that a caster could use to get a direct Brinell reading. There were a few tools one could get to do a comparative measurement, however they never received wide acceptance. Lead Bullet Technologies, (LBT) changed all that.

Lead Bullet Technologies, (HSC 62, Box 145, Moyie Springs, ID 83845), headed by Veral Smith, began to produce an affordable, accurate Lead Hardness Tester that read in direct Brinell units. The LBT tool is simple and foolproof. All measurements taken in this article were taken with my LBT Lead Hardness Tester.

LBT hardness tester with instruction sheet

LBT has a complete line of products for the bullet caster. The LBT Hardness Tester currently sells for $90.00 direct from LBT.

Lee has recently introduced a Brinell hardness tester. The Lee unit is quite a bit different from the LBT, however the Lee unit sells for much less. While I have not had the chance to try the Lee unit, I look forward to trying one in the near future.

Method of heat treatment	BHN two days after Trt.
Quenched directly from mould	30
Oven heated, quenched	35
No heat treatment	9

On a separate set of 5 bullets, I obtained a rate of hardness aging for the oven treatment. At the initial point right after quenching, the BHN was about 20-22, after one day, the hardness had increased to 30-35 BHN. A final hardness test after 2 days gave a reading of 40-41 BHN. In comparing the test above where the 2 day hardness lists as 35, the most likely reason is that when using only 5 bullets, the quenching water started out colder and remained colder for the length of time I allowed the bullets to quench.

As the above table clearly shows, plain wheel weight alloy can be dramatically improved with the simplest procedure. Oven heated bullets showed the most improvement, however, the use of a household appliance that is also used for food may not be wise. Given that linotype has a Brinell hardness of 22, and the easy method of direct quenching yields a hardness of 30, that is an increase of 26%. A very easily attainable result.

Bullet quenched directly from mould shows a Brinell hardness of 30

Oven heated bullet shows a Brinell hardness of 35

An untreated bullet shows a Brinell Hardness of 9

There is one more point we need to cover concerning heat treatment of cast bullets, that being the sizing of bullets that have been heat-treated. If you were to size .001� below the as cast diameter (which is what is normally done in sizing), it removes the heat treatment of the bullets and softens them. There are a few ways to work around this characteristic.

If your mould casts the correct diameter for what you want, simply size and lube at least .001� above the as-cast diameter of heat-treated bullets. An example would be your .38 caliber mould casts bullets at .357. Use a .358 sizer and the heat treatment should not be affected. If you have a mould that casts grossly oversized bullets, use a two step process by casting the bullets, allowing them to slowly cool to room temperature. Then size and lube them down to the correct diameter that you need. After that, use the oven-quenching method to harden the bullets. Realize, however, that you will need to lube them again after quenching as the heat will remove the lube.

Linotype will allow the most casting ease of all alloys, however, the cost and effort involved in getting linotype hardly justifies the result, when straight wheel weight alloy will surely suffice. By adding a bit of 50/50 (50% tin, 50% lead) bar solder to wheel weight alloy I could have increased the �castablity� of the wheel weights. However, for the purpose of this article I used straight wheel weights. I can wholeheartedly recommend this simple, inexpensive procedure to dramatically improve the strength and hardness of your cast bullets.

WARNING

This article addresses issues that deal with reloading firearm cartridges. The information presented is a result of our experimentation and experience. We offer no guarantee or warranty of any kind on the information presented and you should proceed with caution if you choose to try the techniques or products that we present. Reloading is not an exact science, we have no control over what you do or what you use. Therefore, you assume any and all risk involved.

By Mark Trope