Article #1: "Fit is King"

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 #1: Fit is King

As an introduction to a world where the few, eccentric inhabitants seek finer accuracy and higher velocity from their cast, lead-alloy bullets than what is typically considered "normal" by the majority of rifle shooters, I'm going to present some basic articles relating to the "graduate level" of cast bullet shooting and performance. In these discussions I will attempt to describe the key factors affecting accuracy, the forces antagonistic to accuracy as velocity is increased, and some of the methodology used to defeat these forces to a practical limit. The first item of discussion is what many cast shooters consider the thing most important to perfect, because without it, no amount of work can compensate for its lack: Bullet fit.


What is fit? Due to its elastic and malleable nature, lead alloy bullets tend to get very easily damaged, distorted, bent, crushed, slumped, engraved crooked, scraped, skidded, gas-cut, riveted, or otherwise altered when fired out of a gun, particularly at high pressure and high velocity. Proper fit of the bullet to the rifle enables the case neck, throat, and barrel to support the bullet in the critical location, and through the critical events of the firing cycle so that it leaves the muzzle traveling straight and without any balance-altering deformations that will de-stabilize the bullet and cause it to arc or spiral away from the bore centerline, and thus from the aiming point. Fit can be divided into two separate areas: Static fit and Dynamic fit, and measures must be taken by the handloader through bullet selection, preparation, and loading to ensure that the demands of each are met in order to achieve the best accuracy and performance.


Static fit refers to the way the loaded bullet rests in the gun when the cartridge is chambered. Obviously, the bullet needs to be the correct sized diameter to be held by the case, be slightly larger than barrel groove diameter, and have a nose shape and length that will allow the cartridge to chamber fully without the bullet being seated unnecessarily deep in the case, but there's more to it than just those things if you want the best results. Preparations must be made so that the bullet can be launched absolutely straight into the bore from the case without damage. Essentially, the bullet must be shaped and sized so as to not have anywhere to go except straight ahead when the powder lights and kicks it in the pants. Statically, we wish to achieve as little clearance as possible between the case neck and chamber neck so the bullet base and driving bands cannot shift sideways or rivet (bump up) when fired, we wish the driving portions (or "body") of the bullet that are rearward of the throat entrance when chambered to be at or a half-thousandth smaller than the throat entrance, and we wish the nose profile matches exactly the shape of the throat. It is necessary to build in some degree of clearance in all these areas so that the firearm remains functional (particularly in the case of self-loading rifles, hunting rifles that may need the cartridges to be frequently removed from the chamber unfired, and after some normal fouling accumulation), but essentially the goal is to minimize these tolerances as much as is practicable. The bullet does not have to engage the lands, or touch any part of the throat when chambered, but when fired it needs to be fully supported upon making the "jump" to the rifling. The single most important aspects of static fit are that, when chambered and ready to fire, the bullet is perfectly concentric with the bore, and perfectly aligned with the bore centerline.


Dynamic fit is fit in motion, through rapidly changing states and environments. Take a trip with me on a bullet being fired out of a gun. First, the primer is struck, hopefully not moving the cartridge case or bullet in the process (proper case fit prevents this), and the powder lights and begins to build pressure. Once the pressure acting on the bullet base reaches the point where it is able to overcome the static inertia of the bullet, the static friction of the case neck holding it, and resistance (if any) due to contact of the nose or parts of the bullet outside of the case, the bullet begins to move forward. If the bullet is fully supported through this initial motion, and the static fit aligned it properly before launch, it will move straight without the base riveting or slumping off-center. Now our bullet comes in contact with the throat, (freebore, leade, and rifling) and begins the process of being engraved by the lands and swaged to fit the barrel dimensions. As the bullet is changing shape, it is critical that forces are equal concentrically and that the bullet "pilots" into the bore without getting off-center. Avoiding built-in weak points in bullet design such as sharp front driving bands helps greatly in keeping the bullet centered and supported through the engraving process. Also during the engraving process, the hot, high-pressure powder gas is beginning to really get going and is expanding the case to the limits of the chamber (including the neck) and is attempting to rush around the bullet to seek the lower pressure in front of it. If the miniscule gap between the case neck and bullet, and between bullet and throat are too great it can allow streams of hot gas to literally blast away lead and lube, leaving channels and voids in the bullet which cause all sorts of problems including lead deposits in the first part of the barrel, balance issues, lead streaking down the trailing edge of the lands, excessive lube loss, and generally poor accuracy. As the rising pressure drives the bullet into the bore, the lands cut into the bullet and impart a twisting force to our previously straight-traveling projectile. There is opportunity for further damage if the alloy isn't correct or the pressure peaks too quickly and accelerates the bullet too rapidly because the bullet will tend to "skid" the lands as they yank it into a spin. Hopefully, our bullet has now made it fully into the bore without scraping the throat (due to pressure bump enlarging the base, getting crooked, etc.), getting gas-cut, getting crooked in the throat, slumping the nose due to excessive acceleration, or collapsed somewhere due to a void in the casting and is able now to take the full pressure of the burn as it peaks with the bullet only a few inches into the barrel. The seal of the bullet in the barrel through all this must be immaculate; the bullet must FIT the bore well enough to cause complete obturation from point of engraving to muzzle exit. This obturation, or seal, is maintained through a combination of interference fit with the bore in all radial directions, bullet alloy elasticity, and powder pressure on the base projecting upward through the bullet. The pressure peak now kicks our fully-engraved bullet into overdrive, and it now must withstand not only the increasing linear acceleration, but the rotational acceleration as well which is imparted by the lands. Here, the dynamics are extreme and the bullet must withstand the pressures without the nose setting back or the leading edge of the land engraves failing and creating a massive gas leak along the sides of the bullet (loss of obturation). Eventually, the pressure begins to wane, and the bullet streaks toward the muzzle, where it must exit the crown with the base still, after all that, perfectly square to the bore centerline so that it doesn't allow gas to jet unevenly around the base and cause it to tip upon final exit. This launch is quite stressful, even though it only lasts a few milliseconds, and the many ways it can go awry can easily be seen if all aspects of the launch are considered carefully.


Rifle pressures can exceed 50,000 PSI even with cast bullets, and at those pressures even straight linotype alloy is as fluid as putty. Wheel weight alloy commonly used for or as a basis for bullet metal typically has a yield strength of 15-20,000 PSI, and so deforms very easily at the pressures demanded of it. Our goal is to build our ammunition so that when launched, the bullet is damaged the least amount possible and exits the muzzle true. We attempt to achieve this through proper, minimal-clearance static fit of the whole cartridge case and bullet, selecting alloy that has the strength and flexibility to maintain dynamic fit and obturation (yet not deform) when used in proper combination of powder burn rate and pressure curve, using lubricants that are up to the specific demands of high pressure/velocity/shear forces, and by casting high-quality bullets in the first place.


An acquaintance of mine from another forum was fond of observing "It only matters if it does" with regard to things affecting cast bullet groups on paper. For many folks who are perfectly content with 1,600-1,800 fps and two-inch groups at hundred yards, much of the above need hardly be taken into consideration. But one won't get their .308 Winchester to shoot half-inch, 100-yard groups at 2300 fps and above unless the factors I outlined are understood and managed effectively.


In future articles, I will attempt to further dissect and analyze these factors, and provide explanations where possible of some of the techniques that myself and others are employing to achieve very good results.

Link to article #2: http://www.artfulbullet.com/index.php?threads/how-i-make-chamber-pound-casts.131/

Ian