Article #4: Cast bullet fit and design


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Disclaimer: The following information is the product of my own meandering experience and includes a few original ideas and concepts of my own and some that others have been kind to share with me, and is always subject to change as I learn more. It is in no way intended to be the last word, or even the first, just something that may help. Feel free to share like or contradictory experiences and otherwise contribute to the knowledge fund.

The Basement Articles #4: Cast bullet fit and design

While I dedicated the first article to the subject of fit and have discussed its importance throughout, I have somewhat neglected to explore ways of actually accomplishing it because I felt that the topic deserved a dedicated discussion.

The method of breech-seating a cast bullet, or pre-engraving it with a tight-fitting tool made just for the purpose, has long been used as a technique to achieve the utmost accuracy possible from the launching system. Eliminating the long list of things that can go awry in the course of getting the bullet from the grip of the case neck to safely and straight into the rifling takes care of a large portion of the work with creating accurate, fixed ammunition.

So, the challenge to create accurate, fixed ammunition begins at the loading bench where the goal is to create a system that still gets the bullet into the rifling gently and straight, but does it with gas pressure instead of a seating tool. The handicap is that the cartridge case typically isn't as well-fitted to the chamber as is a custom-made breech seating tool, and that with fixed ammunition the bullet is usually pushed directly with gas pressure instead of a solid, square, fully-supported seating punch. It is our challenge to find ways to pre-fit our cases and bullets and select propellant so that the bullet guides itself into the throat straight and gently. If you think of fixed ammunition as "self-breech-seating" you may more easily visualize the concepts I'm attempting to describe.

Since there must be slight clearances between case and chamber and bullet and throat in order to achieve functionality in the field, and best accuracy is typically easiest with zero-tolerance fit, then we have to make up those tolerances through dynamic fit after the primer is struck. One of the easiest fit points is between the bullet and the throat, where simply matching the profile of the bullet from throat entrance diameter to bore diameter will accomplish the task. All one has to do to achieve clearance while maintaining perfect fit, then, is to seat the bullet in the case so that it is just shy of contacting the throat when the cartridge is chambered. Once the bullet moves a few thousandths forward, it will pilot in the throat like a Morse taper. Once the bullet is locked into and fully supported by the throat, the continuing movement that forces the rifling lands to engrave it will occur evenly all the way around the bullet and keep the nose straight for the remainder of the trip up the barrel; the the powder-powered breech-seating condition is now achieved.

If the bullet's nose doesn't match the throat, has large, unsupported steps between the bore-diameter portion and larger driving bands behind, the nose can "wad up", bend, slump, or get pointed crooked as the engraving process is taking place, particularly if the back part of the bullet isn't supported straight in-line with the throat and therefore allowed to be steered off course as tolerance between case neck and chamber neck is taken up by burning gas pressure. The un-supported mass of the nose, the very tip and parts smaller than bore diameter, or between the lands, should be minimized for best results at higher velocities, which is why most cast bullet designs favor a small, flat tip or blunt, round nose. The more pointed the bullet, the more unsupported mass there is to slump or distort upon firing, or to magnify aerodynamic inconsistencies from the slightest imperfections of alignment during launch.


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Now, on to the part of the bullet NOT in the throat yet, but soon to come. Coaxing the body portion of the bullet from the case neck to the bore without distortion is more difficult than the nose, both statically and dynamically. Like I mentioned in earlier articles, most rifles are designed for standardized ammunition using copper-jacketed bullets rather than our more delicate, amorphous, cast lead-alloy projectiles. This means clearances can be very large and still function fairly well with jacketed bullets. Also, copper-jacketed bullets create an even more hazardous "pinch" than do cast bullets if there is a static interference between case neck and chamber, so the ammunition manufacturers tend toward minimum specifications and the rifle manufacturers toward the maximum side of things. The result is very sloppy fit of the case to chamber which allows cast bullet damage and destroys good accuracy potential at high speed. A cast bullet, launched at pressures that build to beyond it's yield strength, will smush around and go anywhere there is room to go, and that isn't always where you want, so you have to control the pressure build and provide as much support as possible during launch by reducing clearances and delaying peak pressure.

Most of the time, a hand-loader finds that his brass is simply too thin in the neck to support the bullet straight during a high-pressure launch, even with bullets sized to the diameter of the throat entrance. Some exceptions, like most .30-30 chamber necks (except some Savage and NEF throats), most .32 Winchester Special chamber necks, and most of the straight-wall black powder cartridges exist where large enough bullets can be used to take up neck clearance to an ideal point (less than half a thousandth total diameter difference), but most of the modern cartridges present quite a problem in the neck area.

There are basically three options to reduce neck clearance to the desired minimum for accuracy, and a few tricks to cheat around excess slop with some limitations to velocity. The options for reducing clearance are larger bullets, thicker case necks, or smaller chambers. The first requires reaming the throat entrance to prevent lead shaving (rent an appropriate reamer from Manson or Clymer, and specify the desired throat entrance size needed to allow minimum neck clearance with throat-sized bullets and typical brass neck thickness), the second involves forming from another caliber to use thicker, lower portions of the case or simply necking down (which is not always possible); making 7mm-08 out of .30-06 brass and reaming the resulting doughnut out of the base of the neck or making .243 Winchester out of .308 Winchester are examples of ways to get thicker necks. The .30-'06 and .270 Winchester are a couple of examples of being stuck. Even using .400 Hawk Basic brass and going .35 Whelen-.30-'06-.270 Winchester won't make .270 necks thick enough to use with most SAAMI-spec. chambers, so reaming together with oversized bullets or re-barreling the rifle and chambering with a tight-necked reamer are really the only practical options for best results in some guns. A few of the cheats I'll mention here involve the extreme end of things that help even perfectly-fitted cast bullets to achieve better accuracy: Unusually slow-for-cartridge powders which can get the bullet's base out of the neck before pressure builds enough to expand the neck to the chamber walls and cause a big gap around the bullet and remove the neck's guiding support, cushioning fillers that act as a soft piston behind the bullet that is less dynamic than direct gas impingement, situating the bullet so that very little of it is in the case neck to get pushed out of alignment or riveted into the excess space created by the case neck expansion, leaving some flare on the case mouth to help the bullet's base remain centered as long as possible in the firing event before the rest of the neck expands to the oversized chamber neck by gas pressure, very hard case necks, tough, flexible alloy (more on that in the alloy article to come), a combination of some or all of the above, and a few others that will be addressed in later articles when this devolves into witchcraft

There's also a trick for improving accuracy (assuming a good fit has been executed in all other areas) that involves compensating for the built-in taper present in many standard bottlenecked rifle chamber necks, but that is one incantation better reserved for the cauldron at midnight during a full moon.

When selecting a bullet mould for a particular rifle, consider the throat shape and throat entrance diameter (and also if the throat entrance will need to be enlarged significantly due to cartridge neck thickness limitations), amount of bullet desired in the neck, and a slump-resistant nose. A custom design may be necessary to achieve the best results with a particular rifle, and for that there are a few custom bullet mould manufacturers to fill the needs. The throat can also be reamed to a different angle or shape to fit a particular, commonly available bullet mould if necessary, so take all of that into account when focusing on bullet design.

Eventually I intend to add some crude drawings of good/bad fit examples since "a picture is worth a thousand words", as they say.

Link to Article #5: