Fit is King - A Practical Journey

[Dimner]

Over and over again, we all hear the term Fit is King in regards to developing a load with cast bullets The point being you need to make sure that prior to ignition a bullet optimally fits the throat (freebore/leade) area so that it can be aligned and ready to be sent down the barrel. Without an optimal fit numerous issues can arise. Leading, scattered groups, yawing, projectile deformation, just to mention a few.

I have read so many threads and posts on bullet fitting. Most of them are a treasure that deserves a sticky in a forum somewhere. Each one I read, I usually can follow along with about 90% of what is being described. I try and imagine, in my minds eye, a diagram of the round in the chamber and the bullet in the throat. Then try to apply it to my own bullet choices. This is where I always fail.

What I usually end up doing is a shotgun approach to bullet fit. Try a bunch of different mold profiles that I own, see which shoots best. Usually this corresponds to a decent, but not optimal, fitting bullet. But you know what would be cool? Being able to start with one bullet and a reasonable amount of confidence that the bullet was going to fit well. That's what I am looking to do in this thread. It comes down to a simple statement.

How does one achieve optimal bullet fit?

I honestly do not know the best way, I'm hoping you all can help me find it.

What I would like to do in this thread is take a journey on how to achieve the optimal fit. I hope that those on this forum help me with the technical terms and wording of what I am doing. To bridge that gap between the abstract language that explains Fit is King to the practical techniques, examples and advice on coming up with an optimal fit in what ever firearm you are casting bullets. So someone in the future can use this to give them a guide on how to do it for themselves. I plan on going all the way from basic terms to posting actual result targets.

It's probably going to end up being a lot more involved than I suppose, but I'm ready for the challenge. So where to begin? Well... as much as I want to just jump right into bullet profiles, throat geometry, and a bunch of stuff I can barely articulate in words... I think first we need to talk about the basics. Terminology, definitions, and the key parts of the system (for lack of a better term) that is involved with optimal bullet fit. So I guess I'll approach this as a work in progress and track each item in parts.

Pt 1: The Basics:
A - Fit is King?
B - Terminology, definitions, synonyms
C - The system ( better term?)

P.S. I'm hoping that you all will help correct me on any and all things I put in this thread. Yeah, I just wrote that. It's the internet, when have people not corrected each other? However, my goal here is 100% accuracy and consistency on this topic in a way that people can understand. Many times I think I understand what is being explained. However, after talking it through with someone, I find that I'm still missing key points. Well more often than not, they find that I'm still not getting it. That's what I will need help with.

So let's start a dialogue

 

[Dimner]

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.

.

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
  • 
• 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?

• Throat types

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
  • Todo: Digest post #8
• 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
  • Driving Bands
  • Nose
• Case preparation
• Bullet Seating Length
  • Jump
  • Touch Lands
  • Jamb

​

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.​

​

 

[Bret4207]

Well, first thing you need to understand is there are 2 parts to the whole fit thing- Static fit and Dynamic fit. Static being everything up to the point the primer lights and Dynamic fit being everything that happens after the primer lights. Might want to break it down into those 2 main sections.

 

[Ian]

Oh jeezus......if this ain't the ultimate can of worms related to this hobby, I don't know what is.

Remember, sometimes we make up terms because historically there haven't been any.

Your drawing is correct. I agree with Bret, and for now, let's stick with STATIC fit. Dynamics is a whole 'nuther thing that really gets off in the weeds; a better topic for later.

People have been trying to nail this stuff down for generations. Problem with getting it right is there's more than one version of right and what works at my house might not work at yours for reasons which can be known but usually aren't because of logistics and flaws in communication. Few of us are true scientists such as Harold Vaughn was and don't have the time, money, equipment, or education at our disposal to make absolute statements about this cast bullet stuff. Empirical, qualitative evidence is difficult to come by outside of velocity and pressure readings. We have computer models and secondary indicators to fill in some gaps. I see many times wrong assumptions made which muddy the waters for ages (a fine example is "running out of lube" causing leading near the muzzle of a rifle. While this may be true to a degree, the more important thing about the anomaly is that it is actually a FIT problem, not a LUBRICATION problem. A dynamic fit problem, to be precise. The pressure drop, alloy strength, and driving side land wear all contribute to maintain bore obturation or not, if not, you get gas leaks, lube blowout, and gas cutting which causes leading). We have to be really careful about assessing the true cause of what we see.

Ok, what is static fit? Let's work on a definition. First, understand that we mean GOOD fit, a bullet to rifle fit which is proven to have good results under very specific conditions. There are many methods to achieve GOOD fit, depending on the firearm, components, and what you're trying to achieve ballistically. So what are the methods? Well...

• Breech seating
• Bore & Groove (typically done with Barlow design, "two diameter" bullets)
• Revolver cylinders (this can actually be grouped under virtually every other method, but there are some unique factors involved).
• Self-aligning (with varying degrees of jump)
• Morse taper fit (exact bullet/throat fit with matching tapers and as little of the bullet in the case as possible)
• Bump fit (think Minie' ball in a musket, or BPCR, particularly paper patched soft lead bullets)
• The Paradox
• The Misfit, or deliberate, careful bullet/throat mismatch (stepped bullet in a cone throat or cone bullet in a stepped throat; one might consider a multi-band, step-tapered Loverin bullet in a cone throat as one example).

So we can break each of these down further by a description of where things touch and don't touch, typical dimensional formulas that work, component formulas (alloys, speeds, powder burn rates, neck tension, bore/chamber proportion, etc.).

I assume the original post will be continuously updated and will become "the article" here. Someone else jump in with some defernitionations.

 

[Ian]

As for C- The System, a person is best to start out measuring and understanding what they have to work with in the first place. If it's a rifle, make a pound cast or other form of accurate chamber cast to include the complete leade; don't waste time "slugging" the barrel. Revolvers need their cylinder throats measured for diameter at the very least and it doesn't hurt to know the leade angle, leade length from breech, groove diameter, how much thread choke, and even forcing cone angle.

What we're doing here is setting up our STATIC fit in certain ways which is like putting props and blocking marks (what's it called?) on a theatre stage. Setting up for the show, which is to be dynamic. If the props aren't in the right place or have moved from what was blocked, bad things happen.

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.

Find what you have and decide what method to use, apply the details (how-to guide) that we'll define for that method, and you have a good starting point at the loading bench. If you get the statics right, the dynamics take care of themselves.

 

[bruce381]

yha I size 1-2 tho over bore and call it good

 

[fiver]

yeah,,,, no.

i'll give you a great example of static fitment.

the benchrest cast bullet shooters cheat.
they have their rifle throated, and then they have a swage die cut with the same throating reamer.
cast lube squish,,, wait,,, load shoot.
yeah they wait.
the alloy hasn't settled in yet when they squish, or they squish and then heat treat and wait, they also use ridiculously slow twist barrels.
1-14 isn't uncommon.

but you ain't got a bench rest rifle or a set of reamers.
so your option is to measure.
pound slug, measure the slug, outline the slug on a piece of paper and mark where you took your measurements.
now you can draw a square sided base that goes back to the neck junction.
move all that over to another piece of paper only bigger and to scale.

now stare at it.
look at it again.
wait till tomorrow and look at it again.
you should see a bullet start to take shape.
now you can start to add features such as a gas check, lube grooves, bulges to mimic the throat, and start figuring out the length and weight.
congratulations your a bullet designer.

 

[Ian]

Damn it, I knew I forgot one! Will add it to the list.

Some methods of fitment support the bullet more than others. John Ardito was famous for throating and bumping an existing military or production rifle (probably .30-'06 though I can't remember now exactly) so as to get the minimum loaded neck clearance. He, like Eagan and many today, used a .5 degree per side straight tapered reamer to ream the throat out enough to accept a virtually conical bullet which was lightly swaged in a "bump" die reamed with the same reamer. The throat would have about a .314" entrance and the bullet's base band and gas check were swaged enough in the die to come out the same. The only part of the bullet that was seated in the neck was the gas check, and the final few thousandths of seating was accomplished by closing the bolt. The idea was to give the bullet 100% support minus the lube grooves, tip of the nose, and the space in front of the gas check which were actually supported dynamically by the lube in the grooves, since it had nowhere to go but become part of the whole, fluid bullet as it drew down through the throat.

He won his share of matches. So did Don Eagan. When you eliminate neck tension and any chance of a crooked start to the bullet, you can do that. The bullets were typically cast of Linotype alloy and due to the size interference and resiliency of the bullet, obturation was good all the way to the muzzle. My problem with this is it makes the rifle useless for anything else, and accurate velocity is severely limited.

I call this method of fitment "Morse taper" fit.

 

[fiver]

when i talk about the 'system' i'm including alloy and powder speed.
alloy and design also go together.
design/alloy, powder speed, and jump go together.
that's a system.

god i wish i could pull up that damn 3 post post i put up over to Jay's place.
it walked through alloy and design choices and why they worked together under the same circumstances.
i'd also love to have that buckshot shot shell workup data i posted throughout the process of working a load over with different alloys and speed changes.
that took me over 2 months to put together.

 

[Ian]

Yeah.

That whole episode and the other one is what prompted me to salvage what I could of my own thoughts and compose the basement articles. They may be raw, rife with grammatical errors, and in stream of consciousness format but I have copies and they're posted here.

 

[Bret4207]

yeah,,,, no.

i'll give you a great example of static fitment.

the benchrest cast bullet shooters cheat.
they have their rifle throated, and then they have a swage die cut with the same throating reamer.
cast lube squish,,, wait,,, load shoot.
yeah they wait.
the alloy hasn't settled in yet when they squish, or they squish and then heat treat and wait, they also use ridiculously slow twist barrels.
1-14 isn't uncommon.

but you ain't got a bench rest rifle or a set of reamers.
so your option is to measure.
pound slug, measure the slug, outline the slug on a piece of paper and mark where you took your measurements.
now you can draw a square sided base that goes back to the neck junction.
move all that over to another piece of paper only bigger and to scale.

now stare at it.
look at it again.
wait till tomorrow and look at it again.
you should see a bullet start to take shape.
now you can start to add features such as a gas check, lube grooves, bulges to mimic the throat, and start figuring out the length and weight.
congratulations your a bullet designer.

Nice! But way, way beyond what 99% of people will ever even think of doing. I don't think most people want that much of an answer. Good points though. I would bet the idea most have is that "Fit" means "Diameter .00____ over bore diameter" and not much else.

 

[Bret4207]

Damn it, I knew I forgot one! Will add it to the list.

Some methods of fitment support the bullet more than others. John Ardito was famous for throating and bumping an existing military or production rifle (probably .30-'06 though I can't remember now exactly) so as to get the minimum loaded neck clearance. He, like Eagan and many today, used a .5 degree per side straight tapered reamer to ream the throat out enough to accept a virtually conical bullet which was lightly swaged in a "bump" die reamed with the same reamer. The throat would have about a .314" entrance and the bullet's base band and gas check were swaged enough in the die to come out the same. The only part of the bullet that was seated in the neck was the gas check, and the final few thousandths of seating was accomplished by closing the bolt. The idea was to give the bullet 100% support minus the lube grooves, tip of the nose, and the space in front of the gas check which were actually supported dynamically by the lube in the grooves, since it had nowhere to go but become part of the whole, fluid bullet as it drew down through the throat.

He won his share of matches. So did Don Eagan. When you eliminate neck tension and any chance of a crooked start to the bullet, you can do that. The bullets were typically cast of Linotype alloy and due to the size interference and resiliency of the bullet, obturation was good all the way to the muzzle. My problem with this is it makes the rifle useless for anything else, and accurate velocity is severely limited.

I call this method of fitment "Morse taper" fit.

Good descriptor.

 

[Dimner]

Thanks for the great start to the discussion fellas. I'm going to start digesting what is here and plugging it into the posts. Ian, you read my mind about what Pt2 (or maybe Pt1 D or E) will be, and that is chamber measurements. I bought a pound of flower of sulphur off amazon so I could do a few chamber casts or rifles and the one pistol I have. (well I have 2 pistols, but they are the same model).

 

[Dimner]

Okay, I've partially digested the posts.

I like fivers idea about a basic approach to determining what bullet to use. I think we should explore that more once I get a few chamber casts prepared. However, for now, I want to stick to off the shelf molds/bullet profiles. Any profile that already exists. That way someone (like me) can look at their own stock of molds and decide what to try first, or if nothing is available decide what to purchase.

 

[popper]

It was relatively easy for me. I started with a design from IIRC - Fiver. Adjusted it to fill the neck (body sizing), freebore and leade with just a couple thous. clearance. Basically as large as will not jam anywhere (semi auto). Ogive was set by Accurate limit on meplat size. When I started PC, modified the nose to include some space from PCing. Removed the lube grooves and set a small groove to collect moved alloy due to bore/barrel sizing. Calculated the amount of moved alloy from front body and from groove to tail. Object to get the base tail as small as possible with a decent rear drive length and GC shank provides a small rear collector groove. Changed total length to adjust for desired weight. 31-165B was start design, ended up with 31-165C (for LR-308). Repeated that process for 300BO for a 145gr bullet. Removed most of the GC shank to get a PB. Same process for 185gr. (and 170gr PB) starting with Micheal's RD design. As Marlin has no freebore, I should have put a step in the nose like the 041 design. They all shoot better than me. For autoloader pistols, just use catalog moulds, I can't shot good enough with irons to tell any difference at S.D. distance. If it feeds and keeps 2-3" @ 25 yds all is good. I did hit a running hog @ 20ish yds, 1" right of my aim point which was the bung hole.
As for alloy, I add some Cu to 4% Sb. LR308 rifle upper spits them out 2700, carbine is 2400, 1:10 twist and carbine does MOA @ 200 yds. Haven't shot the rifle in years. Got to get a few more rnds thru the henry before I try cast in it. Need to send the LR308 down the road or give to GKs as I can't carry it anymore. Pits getting worn out body.
edit: I did slug the Marlin, never done a chamber cast.

 

[todd]

yha I size 1-2 tho over bore and call it good

i agree with that.

the only thing that is high tech is my chrony reader. i've never had a bore scope either.

 

[blackthorn]

Quote ; "I bought a pound of flower of sulphur off amazon so I could do a few chamber casts or rifles and the one pistol I have. (well I have 2 pistols, but they are the same model)."

I have found sulfur casts to be a bit fragile/fracture prone if dropped etc. but the addition of from 10 to 20 percent graphite powder (by volume) added alleviates that. It also seems to assist in removal of the cast from the chamber.

 

[popper]

I got mine at WM and use it on bugs and plants. Did use some once for alloy hardening.

 

[Ian]

Or just use what you have on hand and make a robust, dimensionally accurate pound cast.

 

[Ian]

Alright, let's work on section A, what is fit?

What are we trying to do, in general? Each method of fitment has its own instructions, but they all have one thing in common and I think we can define it. 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. 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.

Add "launch" to your list of esoteric definitions. Launch being 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.