Wednesday, February 17, 2010
17 February 2010
First, a bit of background.
For a given distance from the center of the Earth, the attraction of gravity is virtually constant. If we use the surface of the sea as that distance, the force of gravity will be the same at Bombay, San Francisco or Narvik. There's some minor anomalies here & there -- there are a few crazy places on our planet where water actually does flow uphill -- but for all practical purposes we think of gravity as constant because it usually is.
If you raised a mass some distance above that surface then released it, it would take a certain amount of time to fall to earth. In fact, even if you threw the mass from you it would still fall to earth in the same amount of time, so long as your throw was perfectly horizontal. (If your throw happened to be slightly upwards then the object would take slightly longer to reach the earth.) Indeed, rather than simply dropping or throwing the object, let's say you hurled the object from you as fast as a speeding bullet or a rocket ship, the same rule applies: the object will take the same amount of time to reach the surface, the only difference being how far it travels before touching the ground.
I've mentioned this here because understanding the gravitational constant is fundamental to understanding the details of ballistics. The gravitational constant dictates the flight path of your bullet. To have the bullet fly straight to the bullseye, you want it to fly there as fast as possible, since the shorter the flight time, the less time gravity will have to influence the downward motion of the bullet. But the faster the bullet travels, the greater the chance it won't travel in a straight line. Aerodynamic forces will cause the bullet to tumble. Before we can take advantage of firing at a higher velocity we need to come up with some method of stabilizing our bullets. The way we do that is to spin the bullet, taking advantage of Newton's laws of motion, specifically,the fact that a body in motion will tend to remain in motion until something comes along and changes it. So how fast is our bullet spinning? And just how fast is it traveling down-range? I thought you'd never ask :-)
If we were talking about a muzzle-loader, it's rifling would be on the order of one turn every four feet or thereabouts, and it's muzzle velocity would be about a thousand feet per second. But if we were talking about a modern, post WWI rifle,the twist would be about one turn every ten inches and the muzzle velocity would be almost three times as fast as the muzzle loader, typically around 2700 feet per second.
One turn in four feet (for our muzzle loader) and an initial velocity of 1000 fps means out bullet -- in this case a lead ball -- is spinning 250 times per second as it leaves the muzzle. That's fifteen thousand rpm. More would be better but even 15,000 rpm is pushing it when dealing with a lead projectile because lead simply isn't very strong. Try to spin it any faster and it will simply shear-off where the lead engages the lands of the rifling. A modern weapon, with a rate of about one turn in every ten inches (0.83 feet) and a muzzle velocity of 2700 feet per second has our modern copper jacketed bullet spinning nearly 200,000 rpm. Now we're talking some serious spin -- and a superbly stable projectile.
With a spin-stabilized bullet, a modern firearm is capable of putting ten shots through the same hole in a target a mile away. But that brand of accuracy is very expensive, with each shot costing several dollars.
The more we practice our shooting... the more bullets we fire... the more accurate we will become. On average, the difference between a marksman and a Life Master is about 250 bullets per week. That is, if it takes you fifty practice shots per day to become ...and maintain... your skill as a marksman, doubling that amount of practice will turn you into a Life Master... an Expert Marksman. The problem here is the cost.
By reloading our own bullets we can reduce the cost per shot from over a dollar per shot to something significantly less, the exact amount depending on the price of the primer and powder. We will re-use the brass cartridge case and cast our own bullets. Even so, the lead needed for the bullets reflects significant cost, so what we'll do is begin with used wheel weights.
A wheel weight, as used for balancing tires, is typicaly 95% to 97% lead. Tin and antimony are used to harden the lead but it is the lead that is crucial here. The velocity of our bullet is a function of its mass and the power of the explosive charge that drives it from the barrel. Once the bullet is free of the barrel, the shape of the bullet has considerable effect but right now we're only interested in what is known as interior ballistics -- the stuff that happens before the bullet leaves the barrel. (Once the bullet flys free, it's characteristics are referred to as exterior ballistics.)
The nice thing about old wheel weights is that they are inexpensive. Indeed, many gas stations and tire shops don't bother to recycle them and may even give them away. But for most of us, paying two-bits a pound is more the norm.
We need to know the weight (or mass) of our cast bullets in order to select a suitable powder charge. This is one of those cases where no guessing is allowed, since an error can result in a damaged firearm or even worse, a damaged gunner.
Once you have a batch of old wheel weights, typically five to ten pounds, you melt them down and cast them into ingots of known volume. By accurately weighing the ingot you can determine what percentage of their weight is not lead. You then re-melt the ingots, adding enough pure lead to correct the mass.
Melting lead or any of its alloys is fairly simple because it melts at such a low temperature -- pure lead melts at 621.5 F, which means you don't need much in the way of smelting equipment to turn wheel weights into ingots. Indeed, a few charcoal briquettes provides more than enough heat to melt ten pounds of lead, although a kitchen stove is more convenient and an electrically-powered smelting pot like the one shown in the photo is the most convenient of all.
Once the content of your lead has been adjusted the metal is typically re-cast into ingots of convenient size, typically of 1/2 lb, 1 lb and 2 pound weight. This makes for easy storage and casting.
I reload ammo for eleven different calibers, not including the balls I use in several black-powder firearms. The distinction here is that cartridges are usually not made up for black-powder weapons. Unlike modern smokeless powder that is virtually inert, chemically, black-powder can be extremely corrosive, making it unwise to pre-load it into cartridges or even into your firearms, until just prior to its being used. But modern smokeless powder can be used safely after twenty or more years.
The 'grain' is the usual unit of measure for the mass of a bullet and there are 7000 grains to a pound. The bullet for a small pistol may weigh less than 100 grains whereas a large slug for a blackpowder rifle might weigh more than 25ogr. The 9x18 Makarov uses a bullet weighing about 95 grains or about 73 bullets to a pound of wheel weights.
To cast a bullet we first need a bullet mold. If you are a machinist you can make your own molds but most most people -- including machinists -- prefer to buy their molds from dealers who offer reloading supplies. In the photos you will see some of the many molds I use. The sprue hole of the mold is pressed against the spout of the melting pot and the lever is raised, allowing molten lead to flow into the mold. You may cast as many as six bullets at a time although two is the most common number.
When the mold is filled you release the lever, allow a few moments for the lead to cool, then give the sprue-cutter a sharp rap with a wooden maul. Cutting off the sprue frees the bullet from any over-flow of lead,whilst opening the mold allows the bullet to drop free. The freshly cast bullets are then collected, ready to be coated with lubricant. The lubricated bullet are passed through a sizing die which swages them to a precise diameter, such as .357" for 38 Special, or .356" for 9mm Luger. But the 9x18 Makarov bullet is actually 9.22mm or about 0.363". To produce a bullet of the proper size it is customary to begin with a cast slug at least 0.365" in diameter; to lubricate the exterior and to then pass the cast slug through a set of swagging dies to produce a bullet of the desired diameter.