Article #5: Alloy for accuracy and hunting


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The Basement Articles #5: Alloy for accuracy and hunting

This one is really simple for me, because I basically only use three alloys for everything.

Most common alloy is "Kitchen Sink" alloy, usually about like wheel weights or softer, consists of range scrap, battery cable ends, old fishing sinkers, dive weights, depleted type metal, pipe organ scrap of unknown composition, body solder, and pretty much any other random scrap of questionable composition that I can dig up. This is usually air cooled and used for pistol calibers, particularly plinking, cowboy action, or IDPA shooting. I use a Lee hardness tester to guess at the stuff going into it, and adjust as necessary to add or dilute antimony content. Tin is only deliberately added if the mix is obviously short of it through excessive brittleness and poor casting qualities not attributable to some sort of contamination such as calcium aluminum, or zinc. Calcium and aluminum are removed with copious amounts of sawdust during "smelting" where I ladle the alloy and pour it through the smoldering layer of sawdust repeatedly to extract contaminants and lock them into the sawdust ash. If zinc is a known contaminant, it can be removed with sulfur, but that process is tricky and can singe your lungs.

16:1 or 20:1 lead/tin made from a blend of some or all of radiator shop scrap, clay pipe joint lead, x-ray lead, binary body solder, pipe organ 'common metal' (10:1), and some dead-soft sticky wheel weights. I determine composition by hardness tested a day or so after casting. I use this for some revolver loads, and black powder cartridges (with smokeless powder) like 45/70 or 45/90 with or without paper jackets. For short range stuff I just use K.S. alloy.

50/50. This is one some friends told me about just a few years ago, and I've never looked back for high-velocity rifle shooting and hunting. Basically, it's 50% clip-on, automotive wheel weights and 50% soft lead with a bhn below 6. After cleaning and removing contaminating metals with sawdust flux (this gets out calcium, iron, aluminum, minor zinc contamination, and a few other things we don't want in there), the composition ends up being about 1.5-2.5% antimony, less than 1/4% tin, trace arsenic, and a whole bunch of trace other stuff that doesn't matter much. I either run the mould pretty hot (light, satin "frosty" haze forms on the bullets a few minutes after casting) and water quench the bullets in a bucket of cold water, or air cool them and heat-treat them in a convection oven. The end result of making a bullet from heat-treated, "Low Antimony" alloy is a flexible, tough, malleable projectile that is hard enough to withstand the loads of fast-twist rifling and higher velocities, yet flexible enough to bounce back from launch stresses to a certain degree, and finally is malleable enough to expand on game animals without disintegrating upon hitting bone or failing to expand on flesh. Most of the usage of this alloy comes in at or above 2,000 fps with smaller calibers and heat treating, for larger calibers (.45 plus) I typically just air-cool the alloy and shoot it between 1K and 1800 fps. Air-cooled 50/50 comes in around 9-10 bhn after about a month, and water quenched/heat-treated at about 18-21 bhn after three weeks to a month. The time it takes to achieve full hardness, and how much hardness the heat treating adds, can be changed dramatically depending on trace grain-refiner content (arsenic and sulfur are grain refiners that increase the hardness of heat-treated lead/antimony alloy and shorten the time needed for this hardness to take place).

All ternary lead alloys (lead/antimony/tin) harden with age regardless of quenching method due to dendrite formation, or crystal matrix structure, throughout the metal as it cools. This structure continues to evolve for a while after the metal has frozen and reached a solid state, in some cases it's still getting harder after years due to the crystal structure of the interm-etallic bonds slowly growing. Grain refiners, principally arsenic although there are others, break up the gross lead dendrite structure that forms upon initial freeze into finer, more random "trees" and reduce long, continuous shear planes which allow the metal to move and deform under stress more easily. Basically a 1/4% of arsenic in a ternary alloy will make the structure go from ragged, torn up chicken wire to window screen, which makes a stronger, more flexible alloy without sacrificing malleability.

Hunting alloys often need to be pushed a little harder than target alloy to get better trajectory, more energy at longer distance, or to extend the supersonic range so that the accuracy-wrecking subsonic transition isn't a factor. Higher velocities require tougher alloy to handle the stress of the launch without experiencing accuracy-destroying damage, and simply adding more antimony and tin for a stronger alloy (like linotype, for example) comes with the huge drawback of being too brittle to handle the fit dynamics of launch very well, and tends to be either armor piercing or frangible on game animals, depending on whether it hits soft tissue or bone. The heat-treated, low-antimony alloy is a way around all of that since it has the best of both worlds: Strong and malleable. Some friends of mine and I have pushed this stuff pretty hard, and have made a general determination that it can be pushed to about 40K psi and 2400 fps in a .30 caliber before it begins to fail in the gun. It can be modified down to 30/70 or up to 70/30, and anywhere in-between, to custom tailor it to a specific performance range or gun.