Neck Sizing

[Bart B]

Folks not believing nor understanding how a rimless bottleneck case fits the front of the chamber perfectly centered might consider a few things that explains why and how.

First, inspect your bolt action rifle and notice the one or two things that push the cartridge forward into the chamber until it stops after the bolt's closed.

Second, figure out why the case shoulder being forced into the chamber shoulder will center it there. Note the two angles, both shoulder and chamber, are the same for all practical purposes. Such cases do this the same way headspace gauges do and these gauges do center perfectly in the chamber's shoulder when all the way forward without any neck at all.

Third, if you cannot figure out the first two, then measure the clearance around a case neck to the chamber neck when the round's chambered and pushed all the way forward by those one or two external forced putting it there. There's an easy way to do this; cut the barrel off just in front of the chamber mouth, square it up then use a 10X loupe with a scale in thousandths to do the measuring.

Measure the diameter of a resized case neck (to the nearest .0001"), then compare that to the measured diameter of the chamber neck at the same point, you may be surprised to see how much clearance there is. If there's clearance, then there's no way that area on both the case and in the chamber will do anything to center the case neck in the chamber neck.

Then you may be able to understand why a .243 Win. cartridge will fit a .308 Win. chamber such that when fired, its case neck and therefore the bullet, too, is perfectly centered in the bore. Even if its body diameters are .006" smaller than the chamber and there's .006" head clearance due the case headspace being .006" shorter than chamber headspace.

Some folks have been sizing fired cases this way for decades and getting accuracy equal to or better than what the benchresters do. Sierra Bullets for one; their best match bullets go into sub 1/4 MOA groups in their 200 yard indoor range in Missouri. I've seen several 10-shot test groups from their 100 yard range in California testing 168's in .308 Win. cases that were in the ones; under 2/10ths inch.

 

[Bullet bumper]

Blah blah blah ! I am so sick of MR know it all .

 

[Bart B]

Bullet Bumper, I didn't think it took much reasoning to figure out that an in-line ejector pushes the round forward such that its shoulder centers in the chamber shoulder after the bolt's closed. To say nothing of the fact that the firing pin does so with much more gusto; the case movement stops with its shoulder hard into and well centered in the chamber shoulder. Oft times the case shoulder's set back a thousandth or so before the primer fires. There's enough clearance between the bolt face and extractor lip for several thousandths play of the case rim to do this.

If this is too much for you to figure out, sorry I stressed your reasoning abilities past their limit. There's no doubt other areas of parts fit and function in a rifle you can figure out then know a lot about. To your credit, your were on the right track mentioning the "crush fit" idea for centering the case shoulder in the chamber shoulder. But you forgot, or didn't know about, nor could not understand the force on the case from an in-line ejector or the firing pin.

 

[Mikecr]

I don't believe you Bart
I checked and none of my cartridges change in headspace on primer strike & firing.
And you're forgetting runout. TIR would just as likely prevent 'perfect centering' of chambering. It would also likely cause a case to flinch into abstract fitting of the chamber on firing.
Do your cases also straighten out(drop in TIR) from primer firing?
NOPE

If your cartridges are changing in HS on primer firing alone, it simply indicates very low shoulder angles, and/or excessive striker settings. This doesn't apply to better designed modern cartridges and correctly timed/tuned actions.

As far as an ejector plunger pushing a case forward from boltface, you gotta know this is an angled push, and counter to a well fitting extractor. Bad for centering.

 

[Bart B]

I don't believe you Bart

Most folks don't. I knew you would post such a comment. If your stuff does something else, fine. But I know what my stuff, as well as that of many others, does. So be a nice boy and take your rant off to someone else.

I checked and none of my cartridges change in headspace on primer strike & firing.
And you're forgetting runout. TIR would just as likely prevent 'perfect centering' of chambering. It would also likely cause a case to flinch into abstract fitting of the chamber on firing.

Do your cases also straighten out(drop in TIR) from primer firing?

No, not any measurable amount. They're already pretty darned straight with no more than .002" runout. The ammo shoots as good as anybody's with that much runout.

If your cartridges are changing in HS on primer firing alone, it simply indicates very low shoulder angles, and/or excessive striker settings. This doesn't apply to better designed modern cartridges and correctly timed/tuned actions.

It happens in all the modern Remington, Winchester, and a host of other actions I've used measuring shoulder setback on .308 Win. cases. .30-06 ones and those sub caliber wildcats on it set back more so. And all with factory spec firing pin springs. Many folks have challenged my comments regarding this only to do their own tests then learn I was right.

As far as an ejector plunger pushing a case forward from boltface, you gotta know this is an angled push, and counter to a well fitting extractor. Bad for centering.

Not in my measurements is it bad for centering. Besides, it's repeatable from round to round as is the extractor's sideways push putting the case pressure ring against the chamber wall opposite it.

My extractors are fitted well enough to do their job and there's a few thousandths clearance between their lip edge and the bolt face when the rounds are chambered; both with external Mauser style as well as the sliding ones on the bolt face in front of a locking lug. This lets the case move forward from firing pin impact without interference from the extractors. I've measured the clearance with micrometers. It varies with case rim thickness.

 

[boomtube]

AZShooter: "Continue to lower FL die till it sizes the brass enough for either a fit with slight bolt closing resistance or one that allows you to close the bolt without resistance but no more. There is a fine line between the two settings. Once you find that spot you can repeat it by using feeler gauges to measure the gap between the shell holder and end of die body."

We can duplicate the die set up that way but we can't precisely repeat the shoulder sizing that way. Case spring back varies by brand and production lot, ditto the normal work hardening as it's recycled. I gave up any rote method of die adjustment decades ago and adjust my sizers to match my chambers each time I load a batch.

What never gets mentioned in how tight we adjust a FL sizer for chamber fit is that it hardly matters to a fine degree anyway. The firing pin impact drives each case fully forward in the chamber and then the primer discharge drives it a tad deeper so rimless case shoulders are always going to be fully forward and centered in the chamber. Pressure will expand the thin necks and fore part (probably before the bullet moves more than a small amount) and lock the case in that full forward position as pressure increases to stretch the base back into contact with the bolt face. Thus "proper" - minimum - FL resizing helps limit case stretch more than it affects acuracy. Making accurate ammo involves much more than neck sizing, "partial" FL sizing or highly precise sized shoulder placement.

Normal neck sizing below the neck's contact with the seated bullet is meaningless.

 

[AZShooter]

You said using a feeler gauge "won't precisely repeat the shoulder sizing that way."

I understand but was trying to help someone that is new to reloading. The feeler gauge will set the die very close if it isn't perfect. If there is an issue with the user wanting to have more bold closure resistance when closing the bolt or no resistance it is a simple matter to rotate the die body a tiny amount. The feeler gauge saves the trial and error of running the die down and making multiple sizings till the fit is achieved.

 

[Bart B]

Normal neck sizing below the neck's contact with the seated bullet is meaningless.

Except that it does center the case neck more precicely on the case shoulder. Not much, but noticeable if you use the case shoulder as the front refernce when spinning cases/rounds for neck/bullet runout.

Which is why some accuracy afficianados lap the neck out of a standard full length sizing die instead of using a full length bushing die.

The feeler gauge will set the die very close if it isn't perfect.

That's right, and lots of folks do that. But there's still the issue of controlling fired case shoulder setback. Two things make the spread of resized case headspace vary.

One's the lubricity of the case lube and how much is put on the case. The more slippery the case is going up into the full length sizing die, the less the press will spring and the case shoulder will be set back more.

The other's how long the ram's kept at the top of its stroke. I've set a dial indicator on the press and watched it slowly move the die down sizing the case a tiny bit more until it stops.

Both of these will easily cause a few thousandths spread in sized case headspace.

 

[boomtube]

Bart: "Except that it does center the case neck more precicely on the case shoulder. Not much, but noticeable if you use the case shoulder as the front refernce when spinning cases/rounds for neck/bullet runout."

I haven't found that to be true and I ONLY use the shoulder as the forward reference point and the extractor groove for the rear.

Sizing necks works less on the thick/hard side so any coaxial neck offset moves that way. Neck sizing all the way to the shoulder doesn't correct that.

 

[Bart B]

Normal neck sizing below the neck's contact with the seated bullet is meaningless.

Except that sizing necks all the way back to the shoulder with a full shoulder contact die ends up putting the sized neck better centered on the case shoulder.

That's why a lot folks just lap the neck out of a standard full length sizing die. And as I remember talking with Neil Jones about the neck-shoulder sizing bushings I got from him, those bushings are made so the complete neck will be sized down better centering it on the fired case shoulder that are also set back.

 

[Bart B]

I ONLY use the shoulder as the forward reference point and the extractor groove for the rear.

I've checked several cases for runout between the extractor groove and the pressure ring about 2/10ths inch in front of the case head. Few have zero runout and the rest vary up to 3 or 4 thousandths. Therefore, it's my opinion that the extractor groove will be off center in the chamber by half the amount of total indicated runout to the pressure ring as the extractor's pushing the case against the chamber wall opposite its place on the bolt face when the bolt's closed.

I use the pressure ring as my rear reference as that's the part of the back end of the case that's against the chamber when the round's ready to shoot. The front of the case at the shoulder's centered in the chamber shoulder as you've mentioned and it's the front reference.

 

[jackthebear]

Thank you Ross for sharing your detailed experience. What do you use for a feeler gauge? And do you turn your case necks for concentricity?

 

[Bart B]

We can duplicate the die set up that way but we can't precisely repeat the shoulder sizing that way. Case spring back varies by brand and production lot, ditto the normal work hardening as it's recycled.

Good point.

In my experience, this was a headache; getting full length sized case headspace controlled to a small tolerance. It varies depending on the spring of the press, how long the ram's held at the top of its stroke with the case full into the die as well as the amount/type of case lube used on the cleaned cases. I tried several cures, but only the one below worked consistangly for me.

I've got the smallest spread in sized case headspace on a RCBS Rockchucker using a 50/50 mix of STP engine oil treatment and Hoppe's No. 9 bore cleaner. After fired cases are decapped and cleaned, they're tumbled in a Thumbler's Tumbler in a foam lined coffee can with a few drops of the homemade lube sprinkled around. Tumbling 40 cases them for as long as it takes to full length size them puts on a pretty uniform amount of case lube. Sized case headspace is typically under .002"; enough for excellent accuracy and case life.

I think if one uses the right Redding competition shell holder of the right heighe, then set the sizing die such that the shell holder just bumped it, sized case headspace could be more precicely controlled. It is when I set my dies that way, but the sized case headspace is much more than .001" to .002" less than fired case headspace and accuracy suffers.