Article #3: Case Prep for Accuracy

Ian

Well-Known Member
#1
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 #3: Case preparation for accuracy

To kick this one off, I'm going to make you think a little. Read the following postulation and answer the question: If the cartridge case is resized fully, and thus significantly smaller than the chamber in which it will be fired, and the bullet fairly close-fitting in the throat, then when the cartridge is chambered, the rear portion will be lying on the bottom of the chamber due to gravity or be pressed to the side by extractor or in some rifles by ejector pin spring pressure. Which direction is the nose of the bullet pointing?

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If you answered "NOT at the center of the muzzle", you would be right and thinking along the lines I was intending. Which way the bullet points before and during "launch" from the case is important to accuracy because if it starts crooked due to a poorly-fitted case having aimed it crooked (or for any other reason like case/bullet runout), the bullet will not make it into the barrel without damage. A bullet that is started crooked will have one side of the nose shoved into the lands harder than the other, or it can get bent or otherwise distorted and when it finally exits the gun it won't fly true.


Thus the importance of fitting the cartridge case closely to the chamber. The case steers the bullet during the initial launch. Copper-jacketed bullets are much more tolerant of crooked launches because their tough skins can self-align them in the throats without too much distortion, and the bases don't typically bend or rivet as easily. This is why sloppy-fitting factory ammunition can shoot a well as it does and be loaded to universally shoot pretty good groups in a variety of different guns with different internal dimensions.


I touched on the importance of reducing loaded neck clearance for best accuracy with cast bullets in Article #2 (making chamber casts) and why reducing that clearance is important to a fully-supported and straight launch of the bullet into the bore, but now let's explore the rest of the case and what we want it to do to help our delicate cast bullets get a good, straight ride down the barrel. The first order of business is to reduce cartridge case-to-chamber clearances to a minimum, and the second is to achieve close fit while maintaining concentricity of the case head with the case body, and both concentric with the neck. Also, the bullet and case need to be geometrically straight in line with each other (minimal runout), and with the bore for best results. If your chamber has been reamed crooked with the bore, my advice is to sell the gun to someone you don't like or have it rebarreled.


For the remainder of this article I'm going to refer principally to the modern, bottle-necked rifle cartridge. The principles can be applied in many ways to straight-wall cases, or tapered cases in both rifles and handguns, even the .45 Automatic.


In order to get the case to fit the chamber tightly, a good fire-forming is necessary. Pistol/shotgun powders, cull bullets, and reactive targets make for fun shooting while generating custom-fit brass. I learned three little tricks help get the case formed concentric with the chamber so that the neck isn't lopsided and one side of the body back by the case head isn't bulged more than the other. Begin with full-length sized brass or new brass. The first trick is to cut a little strip of cellophane tape about 1/8" wide and wrap it around the case head just ahead of the extractor groove (or rim on rimmed cases). Build up enough tape (usually two or three wraps) to make it a snug, centered fit in the back of the chamber so the case head will remain centered after firing and the body of the case will be expanded evenly all the way around. The second trick is to support the front of the cartridge case so that it is centered in the chamber by belling the case mouth for a slight interference fit with the front of the chamber neck. There will be some loaded neck clearance in most instances, but this reduces it to zero at one point without risking bullet pinch in the neck and excessive pressure. The third trick involves getting the longitudinal alignment correct by keeping the case head pressed against the bolt face so that the shoulder can blow forward to conform to the chamber's headspace dimension. Simply load the cast bullet out far enough to firmly touch something in the throat (ball seat or rifling) when the action is closed. There should be firm pressure required to close the breech in order to resist the firing pin driving the cartridge forward before the primer goes off, which will short-form the case, stretch the body in front of the case head, and set the primer back.


Now that the aforementioned orders of business is achieved by forming the cases to the chamber, and forming them concentrically, the job of the accuracy-pursuing handloader is to keep from screwing up that custom fit at the loading bench. Usually, case necks must be resized a small amount to hold the bullet, and after a few firings (or with some guns, every firing) the case body needs a slight squeeze and the shoulder needs bumping back a thousandth or two for proper ease of chambering or to maintain correct bolt preload. Bolt or breech block preload is important to harmonics, more on that in a future article. So how to size without ruining the formed case? Some people are fond of neck sizing using a bushing die. I don't like this method because slack in the selectable bushing's support die, and lack of accurate case body/head support and guidance with most bushing-style sizing dies tends to push the neck off center. If brass was perfect, and exactly the same temper throughout, it would yield evenly as the neck is sized, but unfortunately this isn't the situation and the neck can slump off-center to the rest of the case when squeezed down by the die. The Lee Collet neck sizing die, though it doesn't support the case head or body, does guide off of the shoulder and the collet support the neck evenly as it is squeezed. Custom, cast bullet-friendly, larger sizing mandrels can be made to further aid in sizing the necks evenly while minimizing the amount of working of the brass (read: excessive brass working=more opportunity to screw up the neck alignment with the case body). But the Lee Collet dies don't bump the shoulder, and don't size the body at all, so it is necessary to occasionally "bump" the shoulders and body back down in a full-length die. What I do most of the time is actually modify a full-length die to do the bare minimum body sizing, neck bumping, and neck sizing all at once, and do it the exact same way every on every case, every time it is reloaded. I'll discuss this more in the "crafting accurate ammo" article, but essentially I use Emory paper, wood dowels, oil, and a drill press to hone the neck and body portions out on a full-length die so that the die gives just enough tension to the neck to hold the bullet, and shim the top of the shellholder so the die can just "kiss" the shoulder and body enough to chamber easily without reducing clearance beyond what is absolutely necessary for function. The handloader, crafting ammunition for an individual gun, will need to determine what particular tooling and methods work best for the application to achieve the necessary close-fit between cartridge case and chamber.


After properly resizing the case, the neck needs to be prepared to accept and hold the cast bullet by at least belling the case mouth slightly and possibly expanding the neck to size for uniformity. This is another area where getting things "right" and being consistent has a great effect on accuracy. It is also an area where tuning and adjustments can be done to find the situation your gun likes best. Generally, I prefer to size the necks just slightly smaller than needed for the final desired tension on the bullet, and then expand them to the final size. I've found that doing this results in more consistency of neck tension among all the cases than outside-sizing alone due to slight variances in neck thickness, temper, and springback. An inside expander mandrel also irons out scratches and grooves and uniforms the inside surface of the necks. Generally, I hone my FL sizer die neck to reduce the neck inside diameter to about .002" smaller than bullet size (remember that I select bullet body size based upon throat entrance diameter), then select an expander/flaring spud such as made by RCBS for cast bullets (or similar, custom tool I make) that is large enough to leave the neck inside diameter about .0015" smaller than the bullet diameter. I use this as sort of a standard measurement of rifle case neck tension because it has worked out to be just right through experimentation with many different guns, but like most things related to cast bullets it is certainly not carved in stone. I also set the depth of the expander/flaring spud so that it flares the case mouth just enough that the gas check of the sized bullet will sit about 2/3 of the way into the case mouth on its own.
 

Ian

Well-Known Member
#2
(Continued)...

Some other details of case prep that I do for accuracy improvement are neck turning to remove high spots (this is essential to getting the back part of the bullet concentric with the bore), trim to uniform length (but leaving as long as possible up to a couple thousandths shy of the end of the chamber), primer pocket depth uniforming, and sorting by neck hardness (this is a "feel" thing determined by press handle resistance when expanding the necks).


Things I don't usually fuss with because I haven't found the need, but you may, are flash hole uniforming and deburring, and sorting by case volume and/or empty weight. When pursuing fine accuracy with cast bullets in my rifles, I find that my time and energy is better spent elsewhere such as uniform fireforming, modifying tools to massage the brass for just the right grip on the bullet and to achieve close tolerances of case/chamber, and getting uniform necks.

Link to Article #4: http://www.artfulbullet.com/index.php?threads/article-4-cast-bullet-fit-and-design.134/