278 lines
20 KiB
Plaintext
278 lines
20 KiB
Plaintext
.he CHAPTER 3 CORBIN HANDBOOK AND CATALOG NO. 7, PAGE #
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ABOUT BULLET JACKETS
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Bullet jackets are the skin of the bullet. They are what makes it
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possible to achieve velocities over 4,000 fps and still have no fouling
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from melted lead in your barrel. But besides elimination of lead
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fouling, the jacket has another important job. It helps control the
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terminal performance of the bullet.
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Bullet jackets are available from Corbin in packages of 250 or 500
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jackets, depending on the caliber and length. Popular calibers are
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stocked in certain lengths that are most useful. Not all calibers or
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lengths are available directly. Some you have to make yourself, by re-
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drawing a more common size. This is done with a Corbin JRD-1 draw die.
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Other calibers can be made from copper, brass, or even steel
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tubing. A reloading press can only use the commercially available
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drawn gilding metal jackets, which range from 0.017 to 0.032 inches in
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thickness depending on the length and caliber. The Mity Mite press can
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form jackets from 0.030 inch thick copper tubing (hard drawn, straight
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tubing, not the soft coiled type). The Mega Mite can handle tubing in
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0.030 and sometimes in 0.049, depending on caliber. The Hydro-press
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can handle anything, from the thinnest copper to the thickest steel
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walls (typically 0.050 steel or 0.065 brass is the heaviest practical
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jacket wall, beyond which you may as well swage solid copper rod).
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From .30 caliber rifle down, it is both easier and cheaper to use
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commercially made jackets and either use them as is, or redraw them for
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smaller or longer jackets. Jackets can expand considerably during the
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core seating operation, to become larger in diameter. Jackets for
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bullet swaging in Corbin equipment are all made several thousandths of
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an inch smaller than the final bullet diameter, so you can expand them
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upward for a perfect, tight fit on the core.
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This is one reason that it isn't feasible to pour hot lead into a
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jacket and make a jacketed bullet. The pressure of swaging is needed
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to expand the assembly to the right diameter inside a die. Another
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reason is that the hot lead would shrink away from the jacket during
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cooling, leaving a loose core that would not stabilize in the rifling.
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In the calibers from .309 to .338, it is possible to use regular
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.30 caliber jackets. Special techniques to expand the jacket evenly
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include seating the core in two short sections, so that the base will
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form properly. This is done on the .338 and .333 calibers, but isn't
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necessary on the .311 and .314 sizes. Using this method, it is even
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possible to expand a drawn .22 Magnum fired case into a nice 7mm (.284)
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bullet! It works best with a rebated boattail die set and with three
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or four short cores seated on top of each other, one at a time.
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The best known jacket among bullet-makers is the .22 Long Rifle
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case used for a .224 or a .243 caliber jacket. Corbin makes a die set
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called the RFJM-22 that turns these rimfire cases into straight-sided
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jackets of the proper diameter. Vernon Speer, Harvey Donaldson, and
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Fred Huntington were a few of the well-known experimenters who used
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this method in the late 1940's. Speer and Huntington both launched
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major businesses from this beginning.
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The process had flaws in those days, because rimfire cases used
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mercuric priming compound. This left the jackets weak and brittle, so
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they fouled the bores and often came apart on firing. Today, non-
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mercuric priming is used in rimfire cases. The jackets you can make
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for yourself not only are as good as any you can buy for most practical
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purposes, but they are free!
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The disadvantage of making rimfire jackets is that the bullets
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have very thin skins -- typically under 0.015 inches -- without the
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thick taper toward the base which commercial jackets have. This means
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that they are excellent for varmint shooting, because if they hit the
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ground they normally blow up and do not cause a ricochet. But they are
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not suitable for high velocity (beyond about 3500 fps they blow up in
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the air), nor are they suitable for serious game hunting.
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You may be surprised to learn that the commercial standard 52
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grain bullet has a jacket that matches the length of the drawn .22 Long
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Rifle case. This is because the first .224 commercial bullets were
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made from such cases! When bullet makers switched to drawn strip
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metal, they kept the traditional length. Thus, you do not need to trim
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your home-made jackets or make excessive weights of bullets to use
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them.
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The jacket material is normally either commercial bronze or
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gilding metal. These alloys are 10% zinc and 5% zinc, respectively,
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with the balance of copper. The zinc is for strength and anti-fouling
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characteristics. A rimfire case is about 30% zinc. It is more
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brittle, but also less likely to foul the bore at normal speeds. By
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annealing the case, you can make it just as ductile as the regular
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jacket.
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A 6mm (.243-.244) bullet jacket can be made from the fired .22
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case (a Stinger or other long case is best). The head is smoothly
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drawn off, leaving a diameter of about 0.219 inches at the end. The
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body of the fired case is about 0.225 inches in diameter. A special
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punch with a flare or bottleneck, like a bottleneck cartridge, is used
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inside the case. The jacket becomes slightly longer as it is drawn,
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and this lengthening forces the mouth partly over the tapered part of
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the punch. It is this tapered mouth that lets you seat a lead core
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into the undersized jacket and expand it to full .243 size during the
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core seating operation. The flared mouth seals the die against lead
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leakage and makes the jacket expand perfectly.
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The .22 WMR case takes a different die (a special type of JRD-1
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die is used) to make a long 6mm jacket. By careful manipulation of
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core weights and seating technique, you can coax this jacket to become
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a .257, a 6.5mm, or even a 7mm bullet! This isn't something for the
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beginner to try, but once you have mastered the basics, it is fairly
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easy to learn.
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Bullet jackets have different wall thicknesses, not only between
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different calibers and makes, but also within the same jacket.
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Commercial jackets have a taper, to control expansion. This means that
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when you seat the lead core into the jacket, the core has to be small
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enough to fit easily into whatever jacket you are using. If you buy a
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set of dies that is made with punches for a certain jacket, and then
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change to another jacket (such as going from a commercial drawn jacket
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to a copper tubing jacket), you will probably need to obtain a
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different core seating punch.
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The core seating punch fits inside the jacket, rather than the die
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itself, whenever you want bullets with the lead seated down past the
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jacket mouth. (Bullets with large lead tips, such as semi-wadcutters
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and lead round nose bullets, are made using a core seating punch that
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fits and seals pressure against the die wall instead of inside the
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jacket.) If you change from a 0.020 inch wall jacket to one with walls
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of 0.050 inch thickness, then you need a punch that is considerably
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smaller in diameter to fit the new jacket. When you order dies, either
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let Corbin supply the right jacket to fit them, or send a sample of the
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jacket you want to use. If you want more than one jacket, remember
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that you may need more than one core seating punch.
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The other part that you may need for different jackets is the core
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swage die. Core swage dies make the core the right size and weight,
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starting with an undersized piece of lead. Reloading press dies don't
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use a core swage for technical reasons. Special swage presses almost
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always use die sets that can have a core swage as the first die of the
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set. When you order a set of dies for jacketed bullets, the diameter
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of the core swage die is important to the diemakers. The core that is
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produced must be small enough to fit inside whatever jacket you are
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going to use. If you later add copper tubing, or change to a thicker
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wall jacket (such as drawing down a larger caliber to get a longer
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jacket for a heavier bullet), it may require another core swage die of
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smaller diameter.
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The right size of lead wire or core mould for a certain caliber
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depends on the jacket you plan to use. Most standard jackets that are
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available from Corbin take standard, off-the-shelf diameters of lead
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wire and core moulds. If you furnish your own jackets, you may need to
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send samples to get a special size made to order. Core size for the
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jacket is not terribly critical: as long as the core fits and the
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weight is about right without being so long that the core sticks out
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the die mouth, you can use any size.
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One exception is that your core shouldn't normally be a press fit
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into the jacket, so that it traps air in the bottom. The short, thick
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half jackets for .38 caliber sometimes fit a bit snugly on the standard
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core, but they don't cause any problem. It is the long, tapered
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jackets of larger caliber handguns and of rifle bullets that create a
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potential problem with too snug a core.
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The problem arises when the core fits so tightly that air is
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pushed into a highly compressed disk at the bottom of the jacket. If
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the bullet is finished with the jacket brought around the nose or
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ogive section, you don't notice any problem. Accuracy is usually good
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because the air is normally quite highly compressed and centered rather
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well. But if you make a semi-wadcutter style bullet (and by that, we
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in the bullet-swaging field refer to ANY style of nose, be it round or
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flat, hollow or pointed, so long as the nose is entirely made of lead
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projecting from the jacket, and the jacket is NOT curved at all to lock
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the core in place), then the trapped air can expand when the bullets
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are brought into the hot sun. Sometimes, the cores will pop out with a
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loud bang and jump harmlessly across the room!
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The answer is to use a core that slips to the bottom of the jacket
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without force. Or, if you want to use a core swage die that is nearly
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but not exactly right, you might want to have the die-maker machine a
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special internal punch with a cavity in the shape of a boattail or
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Keith nose in its end. The cavity would form a mirror image of itself
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in the lead core, so that a section of the core would then fit nicely
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into the bottom of your tapered wall jacket without trapping any air.
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This technique lets you use the same core swage die with several
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calibers and with several different styles of jacket in the same
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caliber.
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The right core mould for heavy wall tubing jackets is much smaller
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in diameter than the right core mould for a standard drawn commercial
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jacket. In fact, the next smaller caliber of mould is normally used.
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For instance, a .30 caliber core mould might be used with a tubing
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jacket bullet in .358 caliber, whereas a .38 caliber mould would be
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used if you were to make commercial jacket bullets with those same
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dies. When you order, we supply the size of equipment needed for
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standard popular jackets that we stock unless you specify otherwise, or
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unless we know that tubing jackets are going to be used.
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Bullet jackets can make a wide range of weights even with the same
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length. The exact weight range for any given caliber and length of
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jacket depends on the ogive shape and base shape, as well as the degree
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of hollow point or hollow base and the lead density used, and the
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thickness of the jacket. There is no single "right" weight for a given
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jacket because of all these variables. But that means you, as the
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bullet-maker, can manipulate the variables and produce all kinds of
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different weights using a limited stock of jackets.
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For instance, in the .44 caliber, a 0.54-inch long drawn jacket is
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very popular with shooters who have 3-die sets, such as the FJFB-3
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type, to make bullets with the jacket curved around the ogive. By
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adjusting the amount of lead used in the jacket, you can make any
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weight from 180 to 250 grains with this jacket. The lighter weights
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have open points, and the heavier weights have more and more lead
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exposed at the tip.
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The bullet-maker who uses a semi-wadcutter die limits himself
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somewhat on the range of weights possible with a given jacket, since he
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cannot take up any jacket length by curving jacket material around the
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nose. He can, however, change the amount of nose by selecting
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different punches, or change the amount of lead used by selecting
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hollow point or cup base punches and adjusting how far he presses these
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into the die. Then, he can follow with the regular Keith or other
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semi-wadcutter type of punch to shape the lead that is moved forward by
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the hollow point punch.
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This technique lets the bullet-maker adjust weight even on semi-
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wadcutter style bullets with the same jacket length and still have the
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same amount of lead showing! A technique not widely known is the use
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of ordinary cornstarch as a filler in the base of the jacket. By
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placing cornstarch in the bottom of the jacket and seating a lead core
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over it, you can produce very high velocity, light weight bullets in
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jackets that everyone else thinks only make heavy weight slugs.
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In the .25 ACP caliber, you can make jackets from fired shotgun
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primers using the SPJM-25 die set. This kit lets you push out the
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anvil and cap, and draw the top hat battery cup into a smooth-sided
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jacket for a 45-50 grain .25 caliber bullet. Jackets for sub-calibers,
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such as the .14, .17, and .20 caliber, can be made from ordinary .224
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commercial jackets using three drawing dies. The jackets need to be
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annealed by heating to a dull red briefly, so that the bottoms won't
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crack out when you draw them to .14 caliber.
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Dies that use larger caliber jackets, such as the sub-caliber draw
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dies, can be made with a pinch trim punch so that surplus material is
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pinched off as the jacket is drawn. In order to pinch trim a jacket,
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there must be a reasonable amount of reduction taking place. It is
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difficult to pinch trim a .38 jacket being drawn to .350, for instance,
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because most .38 jackets begin at .3545 diameter. That doesn't leave
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enough difference for a pinch trim punch to work. But drawing from a
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.308 to a 7mm, or from a 7mm to a 6.5mm, leaves plenty of metal for
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pinch trimming to any desired length.
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You don't have to use a jacket. Many handloaders don't yet
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realize that bullet swage dies can be used with or without jackets, and
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that a swaged lead bullet can be made faster, more precisely, and with
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far greater control of weight and style than a cast bullet. The same
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dies can be used for jacketed bullets or lead bullets. (This doesn't
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necessarily work in reverse: if you buy a LSWC-1 lead semi-wadcutter
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die, designed just to make lead bullets, it won't make fully jacketed
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bullets because the bleed holes in the side of this die would then be
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covered by the jacket.)
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Lubrication on a swaged, smooth-sided bullet is accomplished by
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changing from Corbin Swage Lube to Corbin Dip Lube. Dip Lube is a
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liquid wax that is applied to the core before swaging a lead bullet.
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The pressure of swaging forms a hard, tough film of wax all over the
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bullet. The wax doesn't melt or affect the powder like bullet greases
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do. Since it covers the whole bullet, no lead is exposed to the air or
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to the bore without having some lube between the bore and the lead.
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Naturally, this "liquid jacket", as some people call it, doesn't
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stand up to the torque and heat of high velocity firing like a regular
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jacket would. It does serve well for most shooters using loads up to
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1,200 fps, and cuts the cost of shooting by eliminating the jacket as
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well as speeding up the whole bullet-making process. Corbin Dip Lube
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is available in pint cans or gallons. A sample 2-oz. bottle is
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available as well.
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Re-forming jacketed, factory or military surplus bullets in a
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standard swage die (the point forming die, usually) is also possible.
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There are some cautions and limitations. The bullet must be smaller
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than the final diameter desired, because you cannot expect a .308
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bullet to fit easily into a .308 diameter hole and eject easily after
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reforming. It should be a 0.3085 to 0.3090 inch die in order to use a
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.3080 inch diameter bullet for reforming. Also, there are some minor
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problems with lead coming forward out the nose of a finished bullet
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when you change the ogive shape to reduce the total internal volume.
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But, on the other hand, we have made hundreds of single-die swages
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that turned rather inaccurate military surplus bullets into soft points
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of very good accuracy simply by swaging them backward, so the base
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became the nose and the pointed nose became a solid base! And in other
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cases, we have made 5.56 and 7.62 mm bullets shoot twice as well by
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simply bumping them up half a thousandth of an inch while making their
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open bases more perfect and even. These transformations are quick and
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easy when they can be made to work with a standard die. I would not
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recommend putting a lot of money into tooling specially built for it
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unless you have a tremendous number of surplus bullets to reform.
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Bullet jackets can be made that have partitions, variations in
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thickness (selective heavier base sections), completely closed bases,
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solid copper bases, and multiple jackets stacked inside each other.
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Most of the heavy duty jacket making, using copper or brass tubing and
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such styles as the partition or H-mantle, are done on the Corbin Hydro-
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press. Hand presses and dies made for them do not have the ability to
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produce or withstand the extreme pressures used. Within a more limited
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range, however, you can still make exotic jacket designs by using the
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telescoping jacket idea: putting smaller calibers inside of larger
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ones is a very effective way to control performance.
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A thorough discussion of bullet jackets can be found in the book
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"REDISCOVER SWAGING", and the various technical bulletins published by
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Corbin Manufacturing have further details on making tubing bullet
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jackets, rimfire cases into jackets, and even the use of fired brass
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cases as bullet jackets.
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