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CM-4a Core Mould

CM-3 and CM-4 moldsCM-3, CM-4 Moulds (discontinued)

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E-mail: dave@corbins.com
Corbin CM-4a Core Mould
The raw material for making a bullet usually consists of a lead core, and a jacket, or sometimes just the lead core itself. While lead can be cut from spools of wire, using the Corbin PCS-1 Precision Core Cutter, or the larger PCS-2 Magnum Core Cutter, for those who have a good supply of soft lead, it may be more desirable to cast the cores.

Casting cores is practical from .170 to .700 diameter with the CM-4a Adjustable Core Mould, which makes four lead cores of adjustable weight at one time. This mold replaces the earlier CM-4 and CM-3 versions, which used a steel frame with steel cylinder inserts. The fine-grained iron block of the CM-4a allows for closer hole spacing, eliminates the fragile cylinder inserts, and simplifies the design while providing better casting characteristics. Each mold has four cavities of the same size, and a rest plate that lets you position the four pistons for any weight of core.

The CM-4a mounts to the reloading bench (or, for those who mount a vise on their bench, to a short piece of 2 X 4 inch wood which is then clamped in the vise). There is no need for a mould handle, as the sprue cutter is long enough to push gently with a gloved hand (wear heat-resistant gloves and face protection when handling molten lead). The lead is simply poured into the "bathtub" slot, in the top of the sprue cutter/handle. It runs down into the bores, which are fitted with pistons that rest on an adjustable stop bar. The bar can be set to different heights, to change the volume displacement of the cylinders, and thus make different weights of cores.

The sprue cutter/handle is moved to one side, cutting the sprues flush with the top of the mold block. Then the rest plate is pushed up, which raises all the pistons at once, and pushes the lead cores out the top of the mould. A box placed under the mould catches the cores when they are pushed over, upon closing the sprue cutter/handle. Using an ordinary open top lead melting pot and a good-sized ladle is the best way to pour the lead. Bottom pour pots designed for bullet moulds can be used, but a ladle should be filled and then used to transfer the hot lead to the core mould. The sprue left in the top can be lifted out with a pair of needle nosed pliers, or a bend paper clip with the end projecting into the sprue area so that the sprue solidifies around it.

The CM-4a is available in .170, .185, .218, .247, .275, .312, .340, .365, .390, .430, and .489 standard diameters, and in custom diameters up to .700 for a modest additional cost. For details and help with the correct diameter for any bullet, contact Corbin. Keep in mind that for jacketed bullets, the core size has to fit into the jacket ID. The jacket ID is smaller than the jacket by twice the wall thickness. Also, the jacket OD is usually smaller than the finished bullet by one or two thousandths. The cast or cut wire core has to be small enough to fit into the core swage die, which adjusts the weight, reduces the length, and expands the diameter all in one stroke. This is why for a jacketed bullet, the core mould needs to be considerably smaller than the caliber.

For instance, a .224, .243, or .257 jacketed bullet typically starts with a .185-inch core, which may be swaged to .190, .200, or .210 diameter in the core swage die before being inserted into the jacket. A .308 bullet typically starts with a .247 diameter core, which is swaged to .250 or .251 diameter for insertion into the jacket.

In a lead bullet, without a jacket, the core can be much larger. Usually a lead bullet is swaged in a LSWC-1 die, which combines the weight adjusting bleed holes of a core swage with finishing the diameter, forming the nose and base, all in one stroke. Therefore, while a .357 jacketed bullet might require a .312 diameter core mold, a .357 lead semi-wadcutter bullet would be easier and better made using a .340 diameter core mold. A .429 (44) lead bullet would use a .390 core mold, but a jacketed 44 (.429) would probably use a .365 diameter core mold. If you don't know the diameter of core mold that is required, you can just specify the caliber of bullet and whether it is jacketed or lead, and if jacketed, whether the jacket is commercial (thinner wall) or to be made from copper tubing (thicker wall).

If you are making lead bullets by first swaging a core in a core swage die made for jacketed bullets of that caliber, and then reforming the core in a point forming die, you will of course have to use a core mold that produces a diameter which fits into the core swage die. A die set for making lead bullets without the SWC shoulder consists of a core swage and a point form die, normally skipping the core seater (which is used to put the core into a jacket, and expand both). In that case, the core swage die probably is going to be made just slightly smaller than the point form die, although with lead bullets it is possible to use a same-diameter LSWC-1 die as a core swage, followed by the point form die. All this means that your core mold can be much larger, even though you are using a core swage first, since the core swage is much larger than it would be for a jacketed bullet in that same diameter.

Troubleshooting tips:

  • Use clean lead.
    Reclaimed range lead and wheel weights usually are infiltrated with road dust, dirt, rock dust, and hard silica dust. Most of these impurities float on molten lead, and can be carefully skimmed from the top of well-fluxed lead as a dross. Impurities in the core will scratch the inside of the cylinder bores, making gouges into which the lead will "lock" when it cools. This makes the pistons hard to move, or even freezes them in place. Clean lead removes this source of trouble.

  • Do not beat on the ejector bar or pistons
    Striking the ejector bar (rest plate) or the pistons in an attempt to make a "stuck" lead core eject will usually ruin the piston rod or cause other damage. Never beat upon any part of the mould. If a piston sticks in a cylinder, stop! Turn the mould upside down, remove the rest plate, put a heat-safe hot lead catching box or tray under the mould, and gently heat the stuck cylinder with a propane/air torch (not an oxy-assist or acetylene torch!) until you can remove the piston. Lead smears can be removed from the bore with a wrap of extra-fine 0000 grade steel wool wrapped on a cleaning rod. Stroke the rod in a straight back and forth motion in the heated cylinder (basically, melt out the lead and then wipe the smeared lead away). Do not chuck the cleaning rod in a drill motor and spin it in the cylinder. This will put circular grooves in the ID of the bore, which will further lock the next core into it. Straight lines are not a problem. The core will slide out even if there are vertical lines in the cylinder. Please note that if you pound on the mould to try and get something to move, you will probably ruin it, and this abuse will not be covered under warranty.

  • Use enough heat.
    When casting in the four cavity mould, there is a considerable amount of heat radiation away from the surface area of the mould. It is necessary to use a higher lead temperature than you would with a standard bullet mould, or else the lead will cool off too quickly and result in partially filled bores.

  • Use a large enough ladle.
    Small ladles tend to lose heat quickly, so that the lead cool and block the holes in the mould before it can pour to the bottom. Use a large enough ladle so that there is plenty of reserve heat to keep the lead molten as it pours.

  • Avoid high tin ratios.
    The mould bores will tend to "solder" to a high tin content alloy. Using antimony to harden the lead instead of tin will help prevent this. Casting solder alloys is not recommended.

  • Avoid bismuth alloys.
    Bismuth alloys usually expand on cooling, just the opposite of most metals. If the core expands in the cylinder as it cools, it will try to lock itself into the bore. The only safe way to remove it will be to turn the mould over, remove the piston, and carefully melt the metal out of the cylinder by application of a propane torch to the mold block. Linotype is an alloy containing bismuth, and tends to expand on cooling. It is also too hard for most swaging operations.

  • Make sure the pistons are down before closing the sprue plate.
    If the pistons do not fall back down, check for tin/lead soldering in the cylinders, scratching from abrasive contaminants in the lead, or other reasons why the piston might be hard to move. If the pistons project from the top of the mould and you close the sprue plate, it will damage the piston, which may be bent or enlarged so it cannot drop back through the bore. If this happens, removal of the piston may require machine work to avoid damaging the bore.

Corbin Manufacturing & Supply, Inc.
PO Box 2659
White City, OR 97503 USA

Phone 9am-5pm Mon-Thurs: 541-826-5211
Fax 24-hrs: 541-826-8669
Website: http://www.swage.com
E-mail: sales@corbins.com

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