What goes into making a bullet mould? What should you consider if you want to make your own design? What are the variables involved and what decisions will you have to make in the process?
Mould making materials Traditionally, bullet moulds have been made out of iron or brass, and more recently aluminum has become popular as a result of its availability, ease of machining and high thermal conduct- ivity. But do moulds have to be made from such stuff? Are there better materials out there? What else has been tried?
Iron isn't all that hard, so it machines fairly easily, but the iron carbide inclusions in cast or forged iron make it very abrasive and hard on the cutting edges of the cherries (this is why RCBS uses tungsten carbide cherries
Belding & Mull made many of their moulds out of nickel. This one (for the Himmelwright wadcutter (left)) appears to be made out of
brass. Yankee moulds were commonly machined out of bronze (like this 452423
mould (right)).
to cut their moulds). Brass cuts very smoothly and is very gentle on cherries. Aluminum also machines easily, but the cut faces are not as smooth as those on brass. Aluminum alloys are also more prone to warpage than brass (although this can be dealt with through appropriate stress relieving).
Belding & Mull cut their mould blocks out of nickel. Meehanite (a cast iron alloy) has also been a popular mould making material, as has bronze. There are also a few experimental modern bullet moulds made from fired ceramic, with hardened steel alignment pins. There are a number of
advantages to using ceramics to make mould blocks (excellent thermal stability, very smooth surfaces, ease of manufacture, lack of warpage, etc.), just don't drop it!
In the Field Museum of Natural History in Chicago there is an Inuit bullet mould that was hand-carved out of a single, split Walrus tusk. I stood staring at that display for quite some time, imagining the many long, cold, lonely nights spent carefully splitting, facing, hinging, and shaping that ivory in some remote igloo until the round balls that fell from it were just right for whatever musket that hardy soul used to feed himself with. I have also seen similar handmade Indian bullet moulds made from bone (buffalo, as I recall), and even bullet
moulds in which the cavity was hollowed out from well-worn river-bottom stones (a stone mould with no handles has got to get HOT!).
Bullet moulds can be made out of many materials. Popular
materials include brass (Applegate 45 315 WFN
mould, top), aluminum (Mountain Molds .40 caliber
200 grain Keith-style SWC, middle), and various ferrous alloys (RCBS 40 180 Cowboy
Number of cavities - Historically bullet moulds were single cavity. After WWI, 2x and 4x moulds gradually started to appear, and after WWII became quite popular with individual casters. "Gang moulds" (6 or more cavities, also called "Arsenal moulds" or "Armory moulds") were traditionally used for the high volume, bulk casting needed by law enforcement groups and shooting clubs. These mammoth moulds take a long time to heat up and are of limited utility to a hobby caster who wants to cast only
a few hundred bullets at a time (this is less of an issue with aluminum gang moulds due to their ability to heat up quickly).
Alignment pins - Early bullet moulds had no alignment pins, relying instead on the
massive hinge pin to keep the mould blocks aligned. Later fixed handle moulds from Ideal incorporated alignment pins. Detachable mould blocks must have alignment pins as there is too much free play and too much variability in mass produced blocks; without this alignment
mechanism to make sure the mould faces line up perfectly, your bullets would come out lop- sided.
Venting - Early Ideal moulds (i.e. fixed handle, single cavity) had no vent lines cut in
the mould faces by the factory. The transition was made to interchangeable mould handles/blocks, but the mould faces remained un-vented. In 1949 Lyman introduced 2-cavity mould blocks, and virtually all of the double cavity moulds I have seen have been vented, but there are a few exceptions (e.g. Himmelwright 2x), suggesting that factory cut vent lines came about after this
Examples of an early Ideal single- cavity mould (the 3118 for the .32- 20) and multi-cavity Armory moulds
(an Ideal 7-cavity mould for the 360344 wadcutter).
date. Reviewing the Ideal Hand-books, moulds are not shown as being vented until Handbook number 43 (1964), but no mention is made of this in the text, or when the change was made. The purpose of these vent lines is to allow air to escape as the cavity is filled, allowing the mould to fill out properly and prevent voids in the finished bullet. Virtually all bullet moulds are vented in some way today.
Early moulds relied on the hinge to align the mould blocks (e.g. this early Ideal .38 wadcutter mould). Later on alignment
pins were added (e.g. Ideal 429251).
Aspects of cast bullet design
Every cast bullet design has the same set of variables that can be
tweaked according to the desires of the designer. There is very little "new under the sun" it's really more of a question of refining what is already out there, and playing some subtly mix-n-match games to combine all of the desired features in one bullet (heck, even this sentiment is recycled -- Elmer Keith said the same thing about his Keith SWC's back when he started designing those back in
amount of contact bearing surface employed, the shape, number and location of lube grooves, the location and configuration of the crimp groove, the diameter and thickness of the forward driving band, the length of the bullet's nose, the shape of the ogive and the diameter of the meplat. Sounds like a lot of fun, right?
Bearing surface - Traditionally, handgun bullets have had about half of their length used as bearing surface (rifle bullets generally use more). More recently, there has been a move towards heavier handgun bullets for deeper penetration, and this in turn has lead to longer handgun bullets with more bearing surface. Bearing surface is a good thing in that it makes sure that the bullet stays concentrically aligned within the throat and transitions smoothly from the throat to the forcing cone to become engraved in a symmetric and concentric fashion. The SSK designs are excellent examples of handgun bullets that take advantage of lots of bearing surface (60+ %) and deliver excellent
accuracy. The bottom line is more bearing surface makes for an accurate bullet since it helps to keep the bullet well- centered during engraving and as it travels down the bore (the Loverin rifle designs are another excellent example of how lots of bearing surface contributes to an accurate design).
Originally Lyman's double-cavity detachable mould blocks were unvented, just like all the early single-cavity moulds (e.g. the Ideal 360302 Himmelwright wadcutter mould shown). Later on the double-cavity mould blocks were
vented by the factory (e.g. the Lyman 357443).
Lube grooves - All cast bullets need to be lubricated (see lube chapter), and this lube has to go somewhere. Way back when, 90 degree right-angled grooves were cut into mould designs for this purpose, and if you've ever cast with these moulds you know what a pain they can be to deal with! As bullet metal shrinks, it shrinks towards the geometric center of the bullet, meaning that the driving bands end up "pinching" the mould at the 90 degree grooves, so the bullet holds fast and does not release from the mould readily. Two methods are commonly used to get around this problem: one is to cut these grooves with a slight bevel to them, and the second is to cut round lube grooves. Both
approaches work just fine to provide "pinch-relief", but the rounded lube grooves generally hold less lube than a beveled flat-bottomed lube groove (what Elmer Keith liked to call a "square-cut" lube groove). Usually, this is of little concern since the rounded lube grooves are smaller and more of them can be used to decorate the bullet's bearing surface, resulting in the same overall quantity of lube. The important issue is how much lube is carried in the lube groove(s), and that they be capable of pumping the lube to the bullet/bore interface (see lube chapter).
Some of the early cast bullet designs had relatively little bearing surface (e.g. the Ideal 403168). Designs with
more bearing surface (e.g. the SSK 44 320 TC) are generally easier to get to shoot accurately.
Crimp groove - Originally, handgun bullets had no provision for crimping; they were simply seated to a depth that allowed the case to be roll- crimped on the ogive. Heel bullets were simply crimped on the heel shank. A very few of the early (pre-1900) rifle bullets had crimp grooves, but most did
not. It's important to remember that these plain- based bullets were designed for black powder, or light charges of smokeless powder ("gallery loads"). The recoil impulse of the gallery loads was light enough that bullets didn't move around in the case, and when these rifle bullets were seated on top of a case full of black powder, the compressed powder charge prevented them from being forced into the case when "waiting in line" in a tubular magazine. Thus, the only need for a crimp was to keep a revolver bullet from inching forward under recoil and a roll-crimp over the ogive was usually sufficient for black powder level ballistics. Smokeless powder would change all this. Suddenly handgun cartridge cases had empty space in them, and
velocities were no longer limited to about 900 fps. Beveled grooves dedicated to crimping had been introduced in rifle bullets with designs like the Ideal #3083 (for the Marlin .30-30), and were a natural next step in the evolution of
handgun bullet design. As near as I can tell, the first handgun bullet to contain a beveled crimping groove was the Ideal 313226 (the 98 grain round-nose for the .32 S&W Long). This system worked so well that others soon followed (e.g. 313249, 358311, and 429251). Elmer Keith identified the 358311 as his
inspiration and identified the beveled crimp groove as one of the more
important design features of his SWC designs (his .38-44 Heavy Duty loads and heavy .44 Special loads generated significant recoil and required a strong crimp to keep the bullet from inching forward). Beveled crimp grooves have been standard fare on all revolver bullets ever since (although the angle, depth and length can vary considerably from one design to the next).
The original Ideal 454424 (left, with "square-cut" grease groove) alongside the later Lyman 454424 with a rounded
grease groove. Later Lyman would re-number this to 452424, and at different times has offered that design
with both flat and round grease grooves (right hand photo).
Ogive/meplat - The ogive and meplat play a central role in determining how stably the bullet will fly and how efficiently it works upon impact. But these features also play a role in the internal ballistics of the load as well. How long is nose of the bullet? In other words, how much of the bullet is seated outside of the case? What is the resulting powder capacity? This will have a direct impact on how fast that bullet can be driven and still stay within sensible pressure limits.
The role of the meplat in crushing tissue and leaving a permanent crush cavity is well established; the larger the meplat, the larger the hole it leaves in its wake. This is why hunting bullets (e.g. Keith SWC's, SSK FP's or LBT WFN's) all have flat noses that are greater than half the bullet's diameter. Flat-nosed bullets are simply more effective and more humane killers. What is not
commonly discussed is the role that ogive/meplat play in the aerodynamic/ hydrodynamic stability of the bullet. The dynamics of how a bullet flies through
the air, as well as how it flies through meat is an important consideration when choosing a bullet to hunt with. Does a given design lead to deep, straight wound channels, or does it tend to tumble and veer off in unpredictable directions? Aerodynamics - The meplat and ogive play a significant role in
determining the
aerodynamic stability (and hence accurate flight) of a given bullet design. When a bullet is traveling faster than the speed of sound, there is a high pressure bow wave that emanates from just in front of the meplat, and trails back behind the bullet. The bullet is basically acting as a piston, compressing the air in front of this cone, with somewhat rarified air (partial vacuum) behind the cone, along the bullet's body. (As an interesting aside, high power rifle competitors will commonly "de-tune" their spotting scopes to focus about halfway down to the target in order to be able to read mirage and dope the wind. This also allows them to "see" the bullet in flight and read the trajectory and wind drift in flight and see where the bullet is being blown of course. This conical pressure wave, and the change in the air's
refractive index from the high/low pressure regimes, is what is being observed by these shooters.) Back to the story -- tests have shown that the ballistic coefficient is more heavily influenced by the ogive, than it is by the meplat. The reason for this is quite simple; the drag experienced by a bullet in super-sonic flight is due to the size and shape of this conical bow wave. The surface area of the meplat is actually quite small relative to the surface area of this entire cone, and so the amount of drag actually due to the flat nose of a bullet (again, in supersonic flight) is fairly small. However the size and shape of this conical bow-wave are directly dependant on how easily it can "wrap itself around" the shape of the bullet, and with a shapely ogive the cone angle is smaller, and therefore the size of the conical bow-wave is smaller and the bullet experiences less drag going downrange. Likewise, when viewed from the side, the cross- sectional area of the cone is smaller, and since it is the "sail area" of this bow wave that dictates how susceptible the bullet is to wind drift, the more shapely bullet gets blown around in the wind less because it has a smaller sail. The combination of these two factors, less drag (hence greater retained velocity, and shorter time of flight) and smaller bow-wave cross-sectional area (a smaller sail for the wind to blow it off course with) are the reasons why boat-tailed bullets drift less in the wind than do flat-base bullets.
Early revolver bullet designs did not include a dedicated crimp groove (e.g. the Ideal 360271 and 360345 target bullets shown at left). Elmer Keith integrated a beveled crimp
groove into the Ideal/Lyman 358429 (right) and most revolver bullets designed since then have followed suit.
The forward portion of the ogive is thus a very important part of the equation, as it plays a heavy role in shaping the bow-wave and determining how well the nose of the bullet "fits" inside of it. A sharp edge at the meplat/ ogive juncture (such as one would get from a truncated cone, e.g. the Gordon Boser or Lee SWC designs) leads to a situation where the only stabilizing
influence this bow wave can have on the bullet is through this perimeter around the edge of the meplat. By placing curvature at this juncture and making the
ogive radiused, the bow-wave is able to wrap around the nose of the bullet, leading to a contact surface instead of just a contact edge. Why is this
important? Well, this bow-wave acts as a dampening agent to damp out any yaw that the bullet might experience. Ever hear of a bullet "going to sleep"? That's just a reflection of how long it takes for any yaw inherent to the launching of a given bullet to be
damped out by this mechanism (in combination with a couple of other factors). The efficiency of this damping mechanism is basically proportional to the amount of surface area that the pressure wave can act upon. In the case of the contact edge, there's relatively little that the bow wave can do to stabilize this yaw, but with a radiused contact surface this damping mechanism becomes much more efficient.
Hydrodynamics - Clearly, a large flat meplat results in greater ability to crush tissue
upon impact, however, it is also well established that extremely large blunt meplats (e.g. wadcutters) are aerodynamically unstable and prone to tumbling upon perturbation. It is important to recognize that while the meplat
determines the shape and nature of the wound channel when a bullet plows through meat, the ogive determines how stably the bullet "flies through meat". All of the arguments given above in the discussion of aerodynamics also apply here. It is interesting to note that J.D. Jones once noted that all of the cast bullets he recovered from big game animals all looked pretty much the same, and he used that shape as part of his inspiration for his SSK designs. He figured that if that's the equilibrium shape that a bullet achieved after punching
through a critter, then starting it off in that shape should provide a reasonably smooth "flight" through more meat.
A shadowgraph of a supersonic bullet in flight. Note the bow wave -- how the ogive of the bullet interacts with this bow wave can have a significant impact on the stability of the bullet's flight. Note also the smaller pressure waves emanating from the grease grooves.Photo courtesy of: http://www.efluids.com/ efluids/gallery/gallery_pages/bullet
shadowgraph.jsp
Optimum meplat diameter - OK, so we know that a big meplat is a good killer, but that too much meplat makes a bullet unstable in flight. How much meplat is too much meplat? Let's look at a few successful designs for some guidance here. Elmer Keith started off with a meplat diameter of 65% by borrowing Heath's ogive and meplat for the Ideal 429336. Keith then used 75% on his 452423, then settled on 68-70% for his 454424, 358429 and H&G #258 (.41 Magnum). J.D. Jones has used 70-75% meplats for his SSK designs. The