Excerpt from Fouling Shot 268, Nov-Dec 2020 - By Steve Hurst, Tucker, GA
For many years the process of making bullet molds used a cutter called a cherry. You made the cherry in the shape of the bullet. The mold blocks were then closed around the spinning cherry and the mold cavity cut was a replica of the cherry. As each mold was cut, the cherry wore a little. The cherry had to be sharpened periodically, the cavity and the bullets it produced got smaller and smaller until the cherry was useless. Then a new cherry was made, it might not be quite the same, sometimes this was intentional and sometimes it was just a different machinist. The same mold ordered next year might be very different in both size and design.
Since the cherry wore with use, the common practice was to make the cherry as large as possible. The cherry would last longer, but the large bullets had to be sized. This was rationalized as being good so the bullets could be sized to the correct diameter. Unfortunately most lubricator sizers did an inferior job due to poor alignment, and early sizing dies were improperly designed. Sizing always damages a bullet and the poor quality of lubricator sizers made this worse. This was demonstrated many times, but the constraints of the manufacturing process prevented any solution.
The design of commercial molds had to fit all firearms and be useable with any alloy. Bullet bodies were usually oversize and bullet noses undersize. Loading manuals were not much help, they instructed to size bullets to the standard groove diameter, measuring this was seldom mentioned. Recommended loads were frequently far too heavy. The result was the general belief that cast bullets were inaccurate; this was not true.
Since the cherry wore with use, the common practice was to make the cherry as large as possible. The cherry would last longer, but the large bullets had to be sized. This was rationalized as being good so the bullets could be sized to the correct diameter. Unfortunately most lubricator sizers did an inferior job due to poor alignment, and early sizing dies were improperly designed. Sizing always damages a bullet and the poor quality of lubricator sizers made this worse. This was demonstrated many times, but the constraints of the manufacturing process prevented any solution.
The design of commercial molds had to fit all firearms and be useable with any alloy. Bullet bodies were usually oversize and bullet noses undersize. Loading manuals were not much help, they instructed to size bullets to the standard groove diameter, measuring this was seldom mentioned. Recommended loads were frequently far too heavy. The result was the general belief that cast bullets were inaccurate; this was not true.
Accurate shooting with cast bullets had been going on for years by Schuetzen competitors using custom bullets carefully breach seated in special rifles. This type of shooting was of little help or interest to the millions of commercial rifle owners. A few individuals, through experimentation or just luck, of a good fit between a commercial mold and a rifle were able to develop accurate loads. These loads and rifles were highly coveted and the information on the load development jealously guarded. Shooting matches had money prizes, no sense helping the competition.
Colonel Harrison changed all this with his articles in the American Rifleman in the 1960's. He showed correct dimensioned cast bullets of proper alloy could be exceptionally accurate with appropriate loads. Elmer Keith did the same for revolvers, increasing the power and accuracy and giving birth to truly accurate, powerful handguns. Now the correct dimensions for cast bullets were known. The problem was the dimensions were different for each firearm.
The cherry machining process was still the only one available. Custom mold makers like Hensley and Gibbs had multiple cherries for the same bullet, you could order a specified diameter. Some mold makers would produce a custom cherry, but this was expensive. Custom molds were therefore costly, but the advantage of bullets cast very close to the desired diameter or of special design to fit the gun was worth the cost for shooters wanting maximum accuracy.
Mass-produced bullet molds were greatly improved by the knowledge of the correct dimensions and modern machining methods. However, they still could not provide perfect fit for an individual gun. If a mass-produced mold happens to be correct for your rifle or pistol, you may get superb accuracy. For shooters having guns with nonstandard dimensions, mass produced molds do not work.
Lathe boring provided a way to make custom molds without a cherry. The greater problem was lathe boring required a good machinist to translate the design into a mold cavity, especially for the accurate replica needed for multiple cavity molds.
Computer assisted machining (CAM) has now solved this problem. Now molds can have the exact design and size to give maximum accuracy and velocity potential in any firearm. The size limitations on CAM lathe boring still exist, but CAM can also produce cherries much more exactly and inexpensively. Even small diameter cast bullets have benefited.
The problem now is you the consumer are the designer. You can greatly benefit from the new technology,
Colonel Harrison changed all this with his articles in the American Rifleman in the 1960's. He showed correct dimensioned cast bullets of proper alloy could be exceptionally accurate with appropriate loads. Elmer Keith did the same for revolvers, increasing the power and accuracy and giving birth to truly accurate, powerful handguns. Now the correct dimensions for cast bullets were known. The problem was the dimensions were different for each firearm.
The cherry machining process was still the only one available. Custom mold makers like Hensley and Gibbs had multiple cherries for the same bullet, you could order a specified diameter. Some mold makers would produce a custom cherry, but this was expensive. Custom molds were therefore costly, but the advantage of bullets cast very close to the desired diameter or of special design to fit the gun was worth the cost for shooters wanting maximum accuracy.
Mass-produced bullet molds were greatly improved by the knowledge of the correct dimensions and modern machining methods. However, they still could not provide perfect fit for an individual gun. If a mass-produced mold happens to be correct for your rifle or pistol, you may get superb accuracy. For shooters having guns with nonstandard dimensions, mass produced molds do not work.
Lathe boring provided a way to make custom molds without a cherry. The greater problem was lathe boring required a good machinist to translate the design into a mold cavity, especially for the accurate replica needed for multiple cavity molds.
Computer assisted machining (CAM) has now solved this problem. Now molds can have the exact design and size to give maximum accuracy and velocity potential in any firearm. The size limitations on CAM lathe boring still exist, but CAM can also produce cherries much more exactly and inexpensively. Even small diameter cast bullets have benefited.
The problem now is you the consumer are the designer. You can greatly benefit from the new technology,
but you can also make mistakes. With freedom comes responsibility, you can easily order a bullet which will not fit or will not be accurate.
First: Think! What do you want the bullet to do?
Design decisions have multiple effects some very different and frequently opposite each other. Meplat size influences terminal effect on game, ballistic coefficient and long-range stability. A big meplat gives good knockdown on game but will limit range and maybe stability. Seating depth affects powder space and velocity. You may want shallow seating for maximum powder capacity/velocity, but this can limit engraved length, which may limit velocity. More or larger lube grooves can increase accuracy and velocity potential, but will reduce engraved length, which will limit accuracy and velocity. You must balance the competing factors.
Then: Measure! You cannot have a custom mold made unless you know the dimensions of the gun. If you plan to use the bullet in several guns of the same caliber, the dimensions of the bullet must fit all the guns. This may mean a compromise in accuracy or performance to gain utility.
You must know: The bore and groove diameter of the barrel, type of rifling and the rifling twist. The rifling twist will set a limit on the length and thus weight of the bullet. If you want the bullet to fit in the magazine this may limit the nose length. Measurement must be made at the front and rear of the barrel which may be different. The type of throat and exact measurements must be known.
In a pistol you need to consider the maximum overall length, seating depth in the cartridge and throat length. In a revolver you need the chamber dimensions, chamber lead, ball seat diameter plus the cylinder length measurement.
You should consider: Will it fit in the magazine? Will it stabilize with the twist rate? Will it fit in the cylinder of a revolver? Will it seat so deep in the case the case bulges and will not chamber? Will it feed in a semi auto? Will the base extend into the powder space below the case neck? Will it fit in a dirty gun? Do you need a gas check? A crimp groove? Where should the crimp groove be located?
Some other considerations: shape of the lube grooves, number of lube grooves, depth of lube grooves, lube groove ahead of the gas check or gas check against the base band, plain or bevel base, equal diameter bands or a tapered bullet, nose type secant ogive, tangent ogive with or without a seating shoulder.
Choose your alloy: To get the exact size in a mold, you must specify the alloy. All bullet alloys shrink on cooling. Alloys vary by the amount of shrinkage. Linotype shrinks little, pure lead shrinks more, other alloys are in between. The mold maker needs to know which alloy you will use, the cavity size will be adjusted to give close to the final cast
First: Think! What do you want the bullet to do?
Design decisions have multiple effects some very different and frequently opposite each other. Meplat size influences terminal effect on game, ballistic coefficient and long-range stability. A big meplat gives good knockdown on game but will limit range and maybe stability. Seating depth affects powder space and velocity. You may want shallow seating for maximum powder capacity/velocity, but this can limit engraved length, which may limit velocity. More or larger lube grooves can increase accuracy and velocity potential, but will reduce engraved length, which will limit accuracy and velocity. You must balance the competing factors.
Then: Measure! You cannot have a custom mold made unless you know the dimensions of the gun. If you plan to use the bullet in several guns of the same caliber, the dimensions of the bullet must fit all the guns. This may mean a compromise in accuracy or performance to gain utility.
You must know: The bore and groove diameter of the barrel, type of rifling and the rifling twist. The rifling twist will set a limit on the length and thus weight of the bullet. If you want the bullet to fit in the magazine this may limit the nose length. Measurement must be made at the front and rear of the barrel which may be different. The type of throat and exact measurements must be known.
In a pistol you need to consider the maximum overall length, seating depth in the cartridge and throat length. In a revolver you need the chamber dimensions, chamber lead, ball seat diameter plus the cylinder length measurement.
You should consider: Will it fit in the magazine? Will it stabilize with the twist rate? Will it fit in the cylinder of a revolver? Will it seat so deep in the case the case bulges and will not chamber? Will it feed in a semi auto? Will the base extend into the powder space below the case neck? Will it fit in a dirty gun? Do you need a gas check? A crimp groove? Where should the crimp groove be located?
Some other considerations: shape of the lube grooves, number of lube grooves, depth of lube grooves, lube groove ahead of the gas check or gas check against the base band, plain or bevel base, equal diameter bands or a tapered bullet, nose type secant ogive, tangent ogive with or without a seating shoulder.
Choose your alloy: To get the exact size in a mold, you must specify the alloy. All bullet alloys shrink on cooling. Alloys vary by the amount of shrinkage. Linotype shrinks little, pure lead shrinks more, other alloys are in between. The mold maker needs to know which alloy you will use, the cavity size will be adjusted to give close to the final cast
diameter you specify.
Last: Listen to the experts. Most mold makers are shooters, they know what works. They also get feedback on designs. There is a good chance an existing proven design will do what you want. Slightly modifying a proven design to fit your gun is often the easiest (and safest) method.
Finally, there is still variation! CNC machining is very good but not perfect. Bullet size will also vary with alloy temperature and mold temperature. Different alloy composition will give different sizes and different weights even if the hardness is identical. Do not expect perfection, specifying diameters ± 0.0005 and weight ± 1 grain is not possible. Variations in an individual casting run are larger than this.
It is possible to get a mold made which casts bullets which do not need sizing. This makes using liquid lubricants possible and will save time and expense. However, a change to a different alloy may give bullets undersize for the gun, a sure recipe for poor shooting. As-cast bullets vary slightly in diameter, if you need exact diameter, get a mold which casts slightly larger and size to the desired diameter. Sizing die diameters can be easily varied and will produce a more consistent diameter.
Final control rests with you. It is a wonderful time to be a cast bullet shooter, it is now easy and inexpensive to have a custom bullet mold made. Before most of us had to dream, now we can realize our ideas, but the dream now must face reality. With any authority comes responsibility.
Last: Listen to the experts. Most mold makers are shooters, they know what works. They also get feedback on designs. There is a good chance an existing proven design will do what you want. Slightly modifying a proven design to fit your gun is often the easiest (and safest) method.
Finally, there is still variation! CNC machining is very good but not perfect. Bullet size will also vary with alloy temperature and mold temperature. Different alloy composition will give different sizes and different weights even if the hardness is identical. Do not expect perfection, specifying diameters ± 0.0005 and weight ± 1 grain is not possible. Variations in an individual casting run are larger than this.
It is possible to get a mold made which casts bullets which do not need sizing. This makes using liquid lubricants possible and will save time and expense. However, a change to a different alloy may give bullets undersize for the gun, a sure recipe for poor shooting. As-cast bullets vary slightly in diameter, if you need exact diameter, get a mold which casts slightly larger and size to the desired diameter. Sizing die diameters can be easily varied and will produce a more consistent diameter.
Final control rests with you. It is a wonderful time to be a cast bullet shooter, it is now easy and inexpensive to have a custom bullet mold made. Before most of us had to dream, now we can realize our ideas, but the dream now must face reality. With any authority comes responsibility.