Ian
Notorious member
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.
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.