Pt 1: The Basics:
A - Fit is what now? King?
So let's define
Fit is King a little better.
Broadly, fit is the relationship of measurements between the bullet itself and anything that touches or will touch it inside of the gun, with the emphasis being on tolerance reduction and exact replication or the the repeatability of our desired fit each time a round is loaded. Keep in mind that regardless of method by which the bullet is fit to the gun, the end goal of any specific mechanical fit technique is starting the bullet absolutely straight into the barrel so that it flies true when it comes out of the muzzle.
Bullet fit can be broken down into two categories:
Static Fit which is how the bullet shape relates to the firearm at the moment it is about to be fired, and
Dynamic Fit which is the moving relationship between the contacting surfaces of the bullet and the inside of the barrel, from chamber to muzzle."
And when I say let's, what I really mean is for the love of god, someone with better writing skills can I please look at your paper? Teacher isn't looking. Just this once? Please? Yeah, just that first paragraph that explains Fit is King briefly. Yeah the one that uses all them smarty word things
Thanks IAN!
Draft Definition - Commonly refers to the static fit of a bullet. The fit of the bullet anytime and upto the moment before the primer is lit. A bullet is considered to have a good fit when, under specific conditions, the bullet and firearm work together to achieve optimal results in a repeatable manner. The desired results are up to the user to determine, just as the method of achieving a good fit is upto the user.
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B - Terminology, definitions, synonyms
- Bullet Profiles(ogive):
- Two diameter
- Also known as Barlow design
- Tapered
- Throat - area of the chamber that spans from the end of the cartridge neck up to the rifling lands. Consists of:
- Freebore - Beginning area of the throat that matches the diameter of the rifling grooves. This helps a bullet to fit without interference.
- Leade - Transitional area that tapers from the freebore to the rifling lands
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- Static Fit - which is how the bullet shape relates to the firearm at the moment it is about to be fired
- Dynamic Fit - the moving (ie: after primer is ignited) relationship between the contacting surfaces of the bullet and the inside of the barrel. It ceases to matter once the bullet has departed the muzzle
- Pound Cast - A form of chamber casting using a fireformed case and a lead slug seated in the neck of the cartridge. A rod is then inserted into the muzzle end of the firearm and tapped against the lead slug. The slug expands to the dimensions of the throat and possibly bore, providing a physical method to determine chamber measurements.
- Sulphur Cast - A form of chamber casting where flower of sulphur is melted and poured into a chamber that is plugged a few inches past the throat. When cooled, the casting can be removed and used to determine chamber measurements.
- Groove Diameter - a value of the firearm's barrel measured from groove to groove. Most often measured from a chamber casting or barrel slugging.
- Throat Wear -
- Launch - The period of internal ballistics and where we find most of our challenges with cast bullet accuracy. This is also the part we can directly control if we know what to do and what to manipulate. The first half inch of the launch is where most of the bad stuff happens to our bullets.
Will put these in alphabetical order at a later date when this list is expanded more.
C - The system ( better term?)
What I'm thinking here for the system is a overview of how the chamber, brass, bullet, freebore, leade and bore all live together and for lack of a better term make up a system that will only work optimally when all are considered, known and accounted for. I figure this section may have quite a few diagrams and may be some of the most technical areas in this ... uhh. ... article?
The Impact of Throat Wear
It is also important to understand throat wear. A throat that starts out with an abrupt angle from chamber end into the parallel freebore, then has an abrupt leade to the tops of the lands, will eventually wear into a trumpet-bell shape. Most rifles wear to about the same throat form eventually, no matter how they start out. If you shoot only mild loads with pistol/shotgun powder you may not ever wear the original form enough to measure, but high-powered rifles that see enough high-pressure loads using rifle powders (regardless of projectile type) are going to wear over time and just like when using jacketed bullets and "chasing the throat" as it wears, STATIC fit is sometimes a moving target.
D - Fitting Methods - A brief description
- Breech seating
- Bore & Groove
- Typically done with Barlow design, "two diameter" bullets
- Revolver Cylinder Approach
- Self-aligning
- Uses varying degrees of jump
- Morse Taper
- Bump Fit
- The Paradox
- The Misfit - a deliberate, careful bullet/throat mismatch
- Examples:
- stepped bullet in a cone throat such as a using a multi-band, step-tapered Loverin bullet
- cone bullet in a stepped throat
Way down the road reminder for myself. Practical examples using my Sav24, Tikka T3, CBAR15, and 81BB
BTW... is there a size limit to a single post?
Pt 2 - Your Actual System - Define It:
Now that we have defined the basics and discussed abstracts, the real first work to achieve optimal static fit will be for the user to create a practical definition of the system that be used. We will take actual measurements of the chamber, throat and bore as well as consider the cartridge specifications to determine any limitations. Those are the fixed variables. That is an oxymoron, kinda eh? However, those variables are still set in stone when working with a specific cartridge and firearm. Only mechanical alteration or choosing a new firearm will change those variables. What we also need to define are the ?soft? variables that also have a significant impact on the system. This includes, desired velocity, lubrication method, preferred bullet weight, and end use for the load (hunting, paper punching, long distance, silhouette, etc). Lastly, we need to consider our dynamic variables: alloy composition, bullet profile, bullet/nose sizing, case preparation, powder burn rate this is for dynamic fit, etc. These variables are going to be freely altered to arrive at our optimal static fit.
To the group...please help. Fixed variables, soft variables, dynamic variables. Better terms are out there, any thoughts?
A - The Chamber Cast
B - Cartridge & Firearm Considerations
- Throat Type
- Case Capacity
- Cartridge Loading Method
- Single Shot
- Lever
- Bolt Action
- Semi-auto
- Revolver
C - User Goals
- Desired Velocity
- Lubrication
- Bullet Weight
- End Use
D - Dynamic Variable
- Bullet Profile
- Alloy Compisition
- Bullet Sizing
- Case preparation
- Bullet Seating Length
Todo: Need to fit this example in here somewhere. Popper, post #15, started with
31-165B and altered to
31-165C to account for an alternate lubrication method.
