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.
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!