Chambered Round Alignment to Bore; Neck Sized vs Full Length Sized

[Mikecr]

Conversely, you should be able to get the same pressure/velocity from a necksized load with a slight increase in powder. ...similar to the whole freebore issue.

That's right Richard, and this is also where much of the notions about uniformity 'provided' by FL sizing falls apart.

As Gary eluded, fired cases go to the expanded chamber(greater than cold chamber) dimensions and spring back. When brass has not expanded so far as to yield, it springs back fully. When brass does expand to yield, it does not spring back fully, but to a new balance in dimensions. This is what new brass does no matter what.
That chamber does spring back fully(or you've got a serious problem), so an interference fit is soon left with high pressure loads, leading to the need to counter with base body sizing, and/or opening the back of the chamber for ease of extraction.
This is an issue even with normal/warmish loads for cartridges that don't account for it with reduced body taper, steeper shoulder angles, and coaxial necks. This gets anyone to flat out FL sizing, as implicated by resulting need for trimming.
Fireforming IS sizing. Spring back IS ALWAYS a part of sizing. And yielding cases, which are changing in spring back, are changing in capacity as well. No getting around it.
Now add to this a FL sizing counter for it, with more spring backs, and probability of matching capacities and low runout drops with each cycle.

This murmur of FL sizing cases being the only avenue to uniform cases, often stands without qualifications otherwise. Wildcat/competitive cases and chambers can be utilized with far less need for sizing. Even to a point of ZERO sizing -ever.
I have a 6.5wssm imp chambered in all aspects fitted. I run loads below SAAMI max, and MY max, which gets me easily to 3200fps with 140vlds. But my 28" barrel tune is at 3040fps, which would be ballistically competitive with these bullets.
My brass did not first see a sizing die until the eighth reload, where I began bumping only with a custom body die. At ~20th reload I added tension tweaking of the load via partial NS with a custom Wilson. But other than shoulders, and my fiddling, these cases could live eternal with no base-body sizing. Had I gone to 45-60deg shoulders(I'm at 35deg), I might have mitigated bumping.
I'll go there, and turn a bit thicker necks(but still fitted), with the next barrel.
I hate bumping..
My cases here still match in capacity, and they are not growing, after 30+ reloads.
This gun will never be a 6ppc or even a 30br,, it's a 16.5lb Williamsport LG, but it is reliable for 1/4moa as far as I've been using it. It has potential at 1kyd, if only I had the time/resources.

Now I know 1/4moa is commonly reached with much easier paths, including FL sizing. But I'm learning there are multiple paths, and that nothing considered with sizing stands as true without qualifiers.

Bart, one thing not mentioned by Gary is that expanding cases do so from case mouth first, to neck-shoulder, to shoulder-body, to body and finally base-body. It truly is inverse FL sizing.
And as mentioned, much of it happens before the bullet even moves.
I don't have a chamber-cam to see it, but have read about it and seen that it passes all tests so far. Anyway, this could either complicate or simplify any mental model of bullet alignment with firing. Much as runout does..
I do know that bolt lugs out of square lead to detriment in accuracy.

 

[boomtube]

Neck sizing only assures the case body is snug in the chamber, it assures nothing about the neck and that's what controls bullet alignment and entry into the bore. We may jam the ogive into the lands for better 'bullet alignment' but if the neck isn't both straight AND concentric the base of the bullet will still be offset.

 

[Bart B]

Bart, one thing not mentioned by Gary is that expanding cases do so from case mouth first, to neck-shoulder, to shoulder-body, to body and finally base-body. It truly is inverse FL sizing.

And as mentioned, much of it happens before the bullet even moves. I don't have a chamber-cam to see it, but have read about it and seen that it passes all tests so far. Anyway, this could either complicate or simplify any mental model of bullet alignment with firing. Much as runout does.

I do know that bolt lugs out of square lead to detriment in accuracy.

Yes, cases expand from their front end to the back end; thanks to case walls getting thinner from extractor rim to case mouth.

As only 269 psi of chamber pressure's needed to push a .308 diameter bullet out of the case neck when 20 pounds of force on the bullet's needed, there won't be much expansion of the case at that time. There's probably a microscopic amount of bullet's axis direction changing (whether or not the bullet's already into the rifling or not) while pressure builds up, but not much I would think. But a 6mm cartridge needs 431 psi to push out its bullet needing the same force to do it.

 

[Reelamin]

I must admit that the Bench Rest guys are doing some amazing things with full length sized case bodies. Yes, they spend another four days after full length sizing the case adjusting and making them fit the neck area perfectly...but some of the top dogs actually full lenth size the main body of the case. Yes, that part is corret....but is the rest of the argumen??

Now to the real world...99% of the shooters will benefit with more accuracy if they neck size a fired case to their chamber.

The physics I was talking about are simple....the least amount of initial adjustment in any object being forced down a given path...will result in less variation in that objects movement at the end of it's path.

 

[Bart B]

The physics I was talking about are simple....the least amount of initial adjustment in any object being forced down a given path...will result in less variation in that objects movement at the end of it's path.

OK, then what are the details as to how, in your opinion, fired rimless bottleneck cases neck only resized align the bullet better with the bore when the round's fired than proper full length sized fired ones? Elaborating on the details on that "initial adjustment" you mentioned is critical to support your comment, for example.

What physically happens to both rounds that makes one better than the other after the trigger's pulled? Explain the details for each starting when the firing pin starts moving forward after the sear releases it. Note that pin's got between 20 and 30 pounds of force that's gonna hit the chambered round's primer.

 

[boomtube]

"Note that pin's got between 20 and 30 pounds of force that's gonna hit the chambered round's primer."

And that force immediatly jams the cartridge fully forward into the chamber so hard it typically sets the shoulder back 1-2 thou. The simulteanous explosion of the primer through the restriction of the flash hole blows the primer cup firmly back against the bolt face; at that point, and no matter how the case was sized, the case is firmly held between the bolt and cone of the chamber shoulder.

Bullets move fairly easily but inertia holds it momentarily as case pressure rapidly ramps up - it's been theorized that the neck and forward part of a case will have released the bullet and is firmly blown against the chamber before the bullet enters the rifling even if it was seated well off the lands. If so, the bullet initially 'hangs in space', certainly the base does, as it enters the bore proper, no matter how the case was sized. Thus, how any advantages due to FL vs. neck sized is open to question but it's clear there is no fixed 'best' way to do it.

A big reason bullet runout happens is the inner and outter cylinders of necks are offten misaligned; if the neck is thin on the upper side of the mouth of a chambered round and thin at the bottom shoulder side then the bullet will be pointed 'up' no matter how the case is sized or if the ogive is pushed into the origin of the lands. That's why modest neck turning on cases to be fired in loose fitting factory chambers will reduce bullet runout and it MAY improve accuracy.

 

[Boss Hoss]

Go to any BR match for any "Proof Positive" you may need that neck sized/fired cases are more accurate and centered to the bore. Show me one of them that is shooting FL sized cases. The physics apply to any rifle.

Speedy Gonzalez BUT he makes his own dies as well!! He makes mine for all of my rifles when they are built. :

 

[Bart B]

"Note that pin's got between 20 and 30 pounds of force that's gonna hit the chambered round's primer."

And that force immediatly jams the cartridge fully forward into the chamber so hard it typically sets the shoulder back 1-2 thou.

Finally, someone's mentioned how they think the cartridge moves in the chamber when the firing pin strikes it.

Anybody disagree with this?

 

[Trickymissfit]

Neck sizing only assures the case body is snug in the chamber, it assures nothing about the neck and that's what controls bullet alignment and entry into the bore. We may jam the ogive into the lands for better 'bullet alignment' but if the neck isn't both straight AND concentric the base of the bullet will still be offset.

I did some checking on just how well a fired case fit the chamber in three different rifles of mine years back. I swabbed the inside of the chamber with high spot blue, and coated the brass with red lead. Then carefully put the cases back in the the chamber and closed the bolt (Remington, two Savages, and a Winchester). Al chambers were either custom cut or national match no turn chambers. What really surprised me was that there really was little contact on the case body O.D.! But on the otherhand there was plenty of contact at the shoulders (probably 75%). The greater the shoulder angle the greater the contact area marked. Thinking I might have had too much red lead on the brass, I completely cleaned them, and wiped them with a Scotch Brite pad for a dull finish. I did the same exact test, and got the same readings.

I learned that I was getting good headspace with my neck sized cases. Later I did the same exact test with full length resized cases that did not move the shoulder much if any at all. I saw the same thing in the shoulders, but even less contact with the O.D. So to take this a step further, I made up several bushings that would contact the case O.D. at the shoulder, midway down, and about a quarter inch above the base of the case. Then I made a long bushing to use with a pair of calipers that would clear the entire case but still contact the bushing. That's how I determined (at that time) that I was actually shrinking the case body too much on the O.D. even if I never touched the shoulders. In otherwords the die was oversizing the case for the chamber size. Or the chambers were over sized for each die. I was going to try and polish out the die to make it size bigger, but never got around to getting it done. Looking back, I'd say that with a tight necked chamber (three of the rifles were) and neck sized cases, the neck itself aided in aligning the cases to the chamber centerline as much as anything I did. But what was really needed was a better die setup from the start.
gary

 

[Trickymissfit]

That's right Richard, and this is also where much of the notions about uniformity 'provided' by FL sizing falls apart.

As Gary eluded, fired cases go to the expanded chamber(greater than cold chamber) dimensions and spring back. When brass has not expanded so far as to yield, it springs back fully. When brass does expand to yield, it does not spring back fully, but to a new balance in dimensions. This is what new brass does no matter what.
That chamber does spring back fully(or you've got a serious problem), so an interference fit is soon left with high pressure loads, leading to the need to counter with base body sizing, and/or opening the back of the chamber for ease of extraction.
This is an issue even with normal/warmish loads for cartridges that don't account for it with reduced body taper, steeper shoulder angles, and coaxial necks. This gets anyone to flat out FL sizing, as implicated by resulting need for trimming.
Fireforming IS sizing. Spring back IS ALWAYS a part of sizing. And yielding cases, which are changing in spring back, are changing in capacity as well. No getting around it.
Now add to this a FL sizing counter for it, with more spring backs, and probability of matching capacities and low runout drops with each cycle.

This murmur of FL sizing cases being the only avenue to uniform cases, often stands without qualifications otherwise. Wildcat/competitive cases and chambers can be utilized with far less need for sizing. Even to a point of ZERO sizing -ever.
I have a 6.5wssm imp chambered in all aspects fitted. I run loads below SAAMI max, and MY max, which gets me easily to 3200fps with 140vlds. But my 28" barrel tune is at 3040fps, which would be ballistically competitive with these bullets.
My brass did not first see a sizing die until the eighth reload, where I began bumping only with a custom body die. At ~20th reload I added tension tweaking of the load via partial NS with a custom Wilson. But other than shoulders, and my fiddling, these cases could live eternal with no base-body sizing. Had I gone to 45-60deg shoulders(I'm at 35deg), I might have mitigated bumping.
I'll go there, and turn a bit thicker necks(but still fitted), with the next barrel.
I hate bumping..
My cases here still match in capacity, and they are not growing, after 30+ reloads.
This gun will never be a 6ppc or even a 30br,, it's a 16.5lb Williamsport LG, but it is reliable for 1/4moa as far as I've been using it. It has potential at 1kyd, if only I had the time/resources.

Now I know 1/4moa is commonly reached with much easier paths, including FL sizing. But I'm learning there are multiple paths, and that nothing considered with sizing stands as true without qualifiers.

Bart, one thing not mentioned by Gary is that expanding cases do so from case mouth first, to neck-shoulder, to shoulder-body, to body and finally base-body. It truly is inverse FL sizing.
And as mentioned, much of it happens before the bullet even moves.
I don't have a chamber-cam to see it, but have read about it and seen that it passes all tests so far. Anyway, this could either complicate or simplify any mental model of bullet alignment with firing. Much as runout does..
I do know that bolt lugs out of square lead to detriment in accuracy.

reason case expansion starts from the case mouth is that that is the weakest point in the case itself. (thinner walls). Also why the botton quarter of the case body expands the least amount.
gary

 

[Trickymissfit]

Yes, cases expand from their front end to the back end; thanks to case walls getting thinner from extractor rim to case mouth.

As only 269 psi of chamber pressure's needed to push a .308 diameter bullet out of the case neck when 20 pounds of force on the bullet's needed, there won't be much expansion of the case at that time. There's probably a microscopic amount of bullet's axis direction changing (whether or not the bullet's already into the rifling or not) while pressure builds up, but not much I would think. But a 6mm cartridge needs 431 psi to push out its bullet needing the same force to do it.

Interesting post, and I just learned something new today. I might add here that when I ran a Remington and a Savage action thru a mechanical CAD program to see what was going on in there I saw several other things than cause pressure spikes along the travel of the bullet besides the powder burn. One of the most interesting things was the rate of twist in the barrel and what the barrel did as the bullet was traveling down the bore. It proved that Issac Newton knew his stuff! As the bullet starts to recieve gas pressure it goes thru a series of pressure spikes and likewise relieves the pressure applied to it more than a couple times.
gary

 

[Trickymissfit]

OK, then what are the details as to how, in your opinion, fired rimless bottleneck cases neck only resized align the bullet better with the bore when the round's fired than proper full length sized fired ones? Elaborating on the details on that "initial adjustment" you mentioned is critical to support your comment, for example.

What physically happens to both rounds that makes one better than the other after the trigger's pulled? Explain the details for each starting when the firing pin starts moving forward after the sear releases it. Note that pin's got between 20 and 30 pounds of force that's gonna hit the chambered round's primer.

almost nobody knows that when the firing pin strikes the case moves forward a tiny little bit if the primer cannot move instead. I find this to be interesting with the concept of seating primers till they crush. But the more contact area (sq. in.) in the shoulder the harder it is to move it.
gary

 

[Trickymissfit]

Finally, someone's mentioned how they think the cartridge moves in the chamber when the firing pin strikes it.

Anybody disagree with this?

I proved this in 2004, after some heated arguments with fellow shooters. But to further add to this, a typical speedlock kit moves the case over twice as far!

gary

 

[Bart B]

almost nobody knows that when the firing pin strikes the case moves forward a tiny little bit if the primer cannot move instead. I find this to be interesting with the concept of seating primers till they crush. But the more contact area (sq. in.) in the shoulder the harder it is to move it.

In my tests of this, a .222 Rem. case had the most shoulder setback, .308 Win. average and a 30 caliber magnum the least.

Note this is the reason H&H put belts on their large caliber elephant rifle cartridges for use in bolt action rifles in the early 1900's; rimless cases made out of big rimmed double rifle cartridges with low shoulder angles moved too far forward in the chamber due to firing pin impact.