Neck tension, expander ball or Not?

[Bart B.]

I think he was feeding me the "tight chamber neck" popular theory they help center case necks in chambers. In spite of the fact that 99.9% of all loaded case necks they're made tight for are are smaller in diameter than those tight chamber necks are.

Sierra Bullets' first ballistic technician (bless his departed soul) tutored me on case to chamber fit for best accuracy. His byline on case to chamber fit is oft times repeated by those who knew him well:

The case has to fit the chamber like a terd in a violin case.

Lots of clearance from body and neck to the chamber. He full length sized all the cases used to test their bullets for accuracy. Their 168 grain Int'l match bullet was the first 30 caliber bullet to frequently shoot in the ones; group after group with 10 shots each at 100 yards.

 

[243winxb]

It takes a" maxi shoulder " to center in my testing, for best accuracy. The unsized neck will fire form to the chamber, after about 3 firings. If i size the neck to "mini" accuracy drops off..

http://s338.photobucket.com/user/joe1944usa/media/2016 Reloading/Bushing neck sizing_zpsdrqj9rw8.jpg.html


The rifle does have a lot of throat erosion, with some extra bullet jump. So maybe not a good test in a 1976 vintage Rem 40X rifle.

Doing your own testing is the only way to know.

 

[243winxb]

The force is enough to set the case shoulder back a few thousandths inch or more increasing case head clearance to the bolt face

Producing slop in the chamber.

 

[Bart B.]

The unsized neck will fire form to the chamber, after about 3 firings.

Did you verify this by measuring both chamber and case necks to 4 decimal places? Or is it just an assumption?

Regarding that "slop" in the chamber caused by case shoulder setback from firing pin force, all rimless bottleneck rounds with headspace less than chamber headspace have that slop before they're fired. It's called head clearance. It's normal and not a bad thing. But the cases aren't loose nor shift back and forth in the chamber while they fire and seal the breech as the bullet goes through the barrel. Even if it's a severely reduced load that doesn't fully expand the case body to chamber limits and the primer backs out several thousandths overcoming firing pin spring force then stops against the bolt face.

Your link says seating of the bullet should expand the case neck .002" minimum for about 45 lbs neck tension. Is that based on the bushing diameter being .002" smaller than loaded rounds neck diameter?

How did you measure how much force the case neck pressed against seated bullets? The industry standard is the force needed to pull the bullet out of the case neck.

 

[Joshboyfutre]

Yeah I had misunderstood what I read I thought that people were talking about taking the expander ball out of a regular die not a bushing die. This all started from breaking the expander ball/deprimer pin on my .223 die and being too sorry to just run it through a neck die after full length to make the neck isn't to tight. Have to admit it's a little emberassing lol good lesson on paying attention to what I read and being careful on just listening to any ole idea I hear lol

Btw Bart B Those groups are pretty damn impressive! Hope I get there someday! I've never even tried a 1000yd, did 800 one time... With a M240 in the Army though lol

 

[243winxb]

good lesson on paying attention to what I read and being careful on just listening to any ole idea I hear

Doing your own testing is the only way to know. Not all info on the interweb is correct.

 

[Bart B.]

Doing your own testing is the only way to know.

More important, test then measure the right stuff the right way then correctly interpret the data.

It helps to understand the mechanical details of how cartridges fit chambers when loaded then how they change that fit when they're fired.

One of the oldest myths that still surfaces today is a loaded cartridge rests on the bottom of the chamber from gravity and stays there when its fired.

 

[Nature Boy]

I'm using a standard Dillon FL die with expander and managing to get really low run out. I must be doing something wrong

 

[Bart B.]

Depending on what runout gauge used, where the two cartridge reference points are relative to each other are and where the measure point is relative to the cartridge reference points, a given cartridge can show a 3X spread in runout across the different gauges available. There are no standards.

Gauges with a single V block for the case body to rest and spin in with a .001" out of round case shape, but the case shoulder and neck are perfectly centered on the body axis, will show .001" runout at any place on their neck or bullet. The case axis moves at right angles to the V block's flats. If the case out of round axis is opposite at one point, the runout can be .002".

None of them align the cartridge to the measuring point like it's aligned in a chamber to the bore when fired.

 

[30Cal]

Those are nice targets, Bart. I must have drawn a faulty conclusion based on a phone call I made over a dozen years back.

 

[Nature Boy]

FYI

Here's a thread I started on neck vs FL sizing

https://www.thehighroad.org/index.php?threads/neck-sizing-308.806500/

Here's another where I'm chasing gremlins in my loads and ended up focusing on run out starting at post #75

https://www.thehighroad.org/index.php?threads/308-load-test.801054/page-3

May be helpful, maybe not.

 

[murf]

It's all based on the Dixie Cup Syndrome. Dropping one cup into another ends up with the inside cup (case shoulder) perfectly centering in the outside cup (chamber shoulder). Their center axes are perfectly in line with each other because their mating surfaces have the same angle.

The 243's shoulder's angle is the same as the 308's. There a lot more clearance from the 243's neck to the 308 chamber's neck than a 308 case has but they're both centered on the cartridge's long axis. Dropping the 243 case into the 308 chamber ends up with its shoulder perfectly centering in the 308 chamber's shoulder. So does a small circle (case neck location) centered on the bottom of that inside Dixie cup have lots of space to the outside Dixie cup's bottom as they're both centered on the same axis. Ditto with 260 Rem, 7-08 Rem and 358 Win chambers.

25-06 rounds do the same thing in 270 Win, 30-06 or 35 Whelen chambers. All have the same shoulder angles.

Meanwhile, standard full length sizing dies squeeze case necks down all the way to their shoulder. They'll size down those "mini shoulders."

the mating surfaces can have different angles and still have the case centered in the chamber. when i fire-form new 220 swift cases in my 220 swift improved rifle (just a mid-level 220 swift load), the resulting group is as good as, and often better than, groups shot using the already fire-formed cases.

so long as the case shoulder angle is shallower than the chamber shoulder angle, the case will auto-center in that chamber.

fyi,

murf

p.s. any case angle may well center itself in the chamber, but i can't prove the steeper angled assumption.

 

[Bart B.]

any case angle may well center itself in the chamber, but i can't prove the steeper angled assumption

Both chamber and case shoulder angles will meet at a round point the same diameter about that 20 spec angle for 308 Win stuff. If the case angle is smaller, they'll meet at a round point at the back of the chamber neck. If larger, at the front of the chamber body. Both ways, case centers.

Get two Dixie cups with different side angles/tapers. Put one into the other then see where they meet.

 

[murf]

that is why it is a good idea to buy premium cases for accurate shooting. if case body thickness varies more than .002", the case neck will be off center more than .001" in the chamber. important? i think so if you are shooting for under a half moa. and you can't get rid of this by neck turning, only by case sorting. probably also why the necks are off-center, not from the expander ball "pulling" the neck off-center, but the die pushing that .002" variance in the case thickness to the inside.

imo,

murf

 

[Bart B.]

if case body thickness varies more than .002", the case neck will be off center more than .001" in the chamber.

Regarding the 308 Winchester, bullets .003" off center by axis or runout in a SAAMI spec chamber will shoot about 1/2 to 2/3 MOA at 600 yards. 300 yards and closer, 1/3 to 1/2 MOA. With otherwise good ammo and good rifle; naturally. The correct leade (throat) angle straightens them up nicely.

 

[murf]

makes sense as the cartridge runout would include any deviation in both the case body and the case neck. i'm guessing a 1.5 degree leade as correct?

murf

 

[Bart B.]

Yes, the 1.5 degree throat of the 308 Win chamber is one reason their bullets shot more accurate than the same ones from a 30-06 chamber with a SAAMI spec 2.5 degree throat. SAAMI 30-06 spec was changed to a 1.37 degree throat several years later. The 308's spec was changed to a 1.75 degree throat. Why? I've no idea. Lesser throat angles straighten crooked bullets better than greater ones do. Some people think they also produce more uniform pressure curves which leads to lower muzzle velocity spreads.

 

[Joshboyfutre]

Is throat angle something a gunsmith can change? For instance my 308 is probably 1.75 (from mwhat u said, dont know for sure). Could I take it to a gunsmith and he ream it out to 1.5? Would It be worth doing to a standard hunting rifle?

 

[Bart B.]

Is throat angle something a gunsmith can change?

For instance my 308 is probably 1.75 (from what u said, dont know for sure).

Who is "u?"

Could I take it to a gunsmith and he ream it out to 1.5?

Yes, but it would increase bullet jump to the lands.

Would It be worth doing to a standard hunting rifle?

I don't think so.

 

[Blue68f100]

You could go possibly larger but not smaller. Smaller would require the chamber to be shortened so it could be re-cut to a smaller size. Which means your going to do some major work if you want to maintain your current bullet jump.