• This Forum is for adults 18 years of age or over. By continuing to use this Forum you are confirming that you are 18 or older. No content shall be viewed by any person under 18 in California.

How much is too much when it comes to neck clearance?

I think we're married to the same woman, if not they gotta be sisters!
Your brass will let you know if it's enough clearance.
I had a clearance issue in my 06' with lapua brass. Chamber neck .340, loaded round was .337.
Now I could've bought different brass to solve the problem, thought about it a lil bit.
I thought it a better idea to buy a used 21st Century lathe, and new cutter head....
I found out I should listen to our wives, it would've saved me $200.
Did you tell your wife you should have listened to her?!?!?:eek:

I know I have read it some where in the past.. but shouldn’t/doesn’t the brass neck spring back closed a little after firing?
 
Did you tell your wife you should have listened to her?!?!?:eek:

I know I have read it some where in the past.. but shouldn’t/doesn’t the brass neck spring back closed a little after firing?
Hell no I didn't tell her!
Woman calls me rain man as it is with all the numbers.

I wound up having excessive preasure at charges 2.5- 3gr back of max.
Velocities were quite impressive, but could not drop a bullet into a fired case.

Now back to counting toothpicks!
 
A lot of the work the case neck stuff came from back in the day when most shooters were using one piece dies in which the IDs of the neck part of the dies were way undersized, so that necks were sized down a lot more than they needed to be, and then the expander ball opened them up by expanding them quite a bit. If you are running a bushing die, or a custom one piece die with the correct neck ID you are working your necks a LOT less than an off the shelf one piece die would. The other thing that happened with those dies was expander balls getting a bad rap, when the real issue was the undersized die neck IDs. If the neck is sized the correct amount, opening it back up a very small amount with either an expander ball in the die or a mandrel in a seperate operation , as you might want to do to uniform unturned necks, does no damage.
 
On my 30 cal hunting rig necks, I don't turn brass and have been happy with the 0.340 necks.

0.3085" + (2 x 0.013") = 0.3345 compared to 0.340 means 0.0055 to 0.0045 loose depending on the brass batch.

Any tighter and dirt and weather can remind you of why mil specs were done the way they were. You could go looser, but I don't think it adds anything to the party.
 
During all the things mentioned during the neck diameter trade-offs, I rarely hear folks talking about the reamer designs in terms of how big of a deal are the dimensions.

Do typical neck diameters take the brass into weird places where the laws of physics are broken, or are they within the dimensions where we can say, no worries and let the functional and accuracy topics take priority? Clearly, in nearly 150 years of cartridge use, brass wasn't the wrong choice, but is it worth a minute to explain why a 0.340 neck is okay on a 30 cal? How good or bad is that much expansion?

The perspective of how the chamber neck diameter "looks" to the designers as an over simplification for the layman takes a few paragraphs and I am stuck with a few minutes, so....

How big of a deal is the diameter of the neck, based on the neck expansion during the shot? Why does it work?

For just a few paragraphs you have to describe the engineering and material property terms for folks who don't live and work in that field. So strain, stress, and tensile modulus are coupled and need to be over simplified for a moment.

Strain in the study of metals is roughly based on a change in dimension. So, some strain is within the elastic limits and it "springs back completely" and some goes into the yield region and you get some plastic flow that tends to stay that way so the sample ends up a little longer.

Don't get confused by the word strain because some strain is recovered and some is not. Clearly when we consider the reamer design, the trade-offs on how far can we allow the neck to stretch (strain) needs to take the material's property limits into perspective.

Material, like the cartridge brass in your neck, has a modulus, or more correctly it has several but we will just make this easy and talk about the tensile (pulling or stretching) modulus for a minute. The way we want to think for this simplification is that the modulus is the spring constant. It is the stress divided by the strain, but only the part within the elastic limits. Go too far and the modulus doesn't mean the same things and the "simplifications" don't apply. My description here is not to be taken literally, it is to say, a way of putting a perspective on where brass and neck diameters fall in terms layman might grasp the dashboard levels for the brass.

When we pull on a regular metal test sample in a stress/strain test in order to study the tensile properties, we plot the dimensional (strain) parts horizontal and the force (stress) parts vertical. As the sample gets pulled, the plotter starts moving on a slanted line and that first sloped region that is the elastic part is pretty straight and then it starts to curve over. The vertical axis is the force (stress) and the horizontal axis is the strain.
brass plot.jpg

We "normalize" the samples for area along the vertical force axis and that makes it force/area = pressure, and we "normalize" the horizontal axis into percent strain by dividing the displaced sample length by the original sample length. There is a standard of plotting a parallel line along that straight part that is offset on the lower axis by 0.2% (0.002) of the length dimension and we establish that the tensile modulus is the slope of that line to the point where it intersects the curve. That is also the point that we call the Yield Strength since it is the start of where that brass doesn't recover. So all that is to explain the vertical and horizontal axis for a simple pull test.

Now to give you an idea of how a reamer diameter sits on that scale. If we take the example of a 0.340 neck for a 30 cal, and simplify the example by saying the hoop strain is like the comparison of the circumference difference between the loaded cartridge and the neck (which is an oversimplification since there is stress, yield, and work hardening already present in a loaded round), the strain is about 1.6% or on that plot it is about 0.016 and goes way past the 36000 PSI yield strength.

Now the weakness of the simplification makes this next simplification untrue, but for the simple example of how small the neck diameter would have to be to stay under the tensile yield curve, we back out what the strain would be to stay below 0.2% and that would be about 0.0007" . In reality there are also other modulus terms that complicate the actual issue with pressure terms that mean you would have to be even tighter then 0.0007" so don't take this as anything like a prescription for a reamer. It is just to show how even the simple tension view would look in terms of brass properties.

Many materials like brass, steel and aluminum have similar looking stress/strain charts, but can still have wild differences in how they respond to work hardening and fatigue. For example, aluminum has no stress under which it will not fatigue, and many steel alloys will last "forever" if you keep under about one fifth of the yield strength. Brass is somewhere in between. So it starts to fatigue and work harden well below that 36000 PSI level. But don't over-think all of this. In most designs, case necks can last so long that the primer pockets give up much sooner. Because the typical pressures involved in modern centerfire designs are above the fatigue and work hardening pressures of cartridge brass, it doesn't pay to try and find a reamer diameter so tight that the necks never harden. That is to say that reality makes all the above an over-simplification just to show how that one stress/strain parameter looks compared to neck sizing.

Hope that helps put a rough perspective on how brass looks on the material charts when we talk about neck diameters and reamers or even sizing dies.
Happy New Year!
 
Good reply. Back when I started loading I ran a simplevFEA on a 308 neck... from formed with .002 tension(.306" ID), and the simple step of seating a .308 bullet pushed half-hard brass past yield. I wanted to test a hypothesis at the time. Expanding during firing would definitely push well past yield and begin work hardening. There is no way to eliminate the work hardening during firing. This is why we anneal.

I received a lesson in neck clearance from a recent 300 BLK build. Cheap top brass brand wouldn't chamber due to the super tight necks. Turned 200 pcs to .011 on my forster trimmer. .004 clearance. Whoever made the stock chamber neck .334 wasn't considering the problems using different brass.
 
Last edited:
@3Sigma agree. Neck clearance is really about being practical while avoiding any unnecessary work.

To be honest, many folks should consider that their FL dies are as big a variable as their chamber reamers.
 

Upgrades & Donations

This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.


Click Upgrade Membership Button ABOVE to get Gold or Silver Status.

You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.


To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.

Forum statistics

Threads
165,835
Messages
2,203,968
Members
79,147
Latest member
tsteinmetz
Back
Top