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Any real experts please chime in!

They are used to measure the variance and thickness of the formed brass neck of a case. You can do the same thing using an accurate caliper, taking measurements around the neck.

I have never used one, but my understanding is that neck thickness is used in a few ways, mostly in the competitive benchrest and wildcat development crowd.

In a nutshell, when trying to achieve extreme conformity when handloading ammunition, neck tension can be important for consistently developing uniform pressure curves during ignition. Making the neck perfectly concentric, and with the same inside and outside dimentions is an attempt to keep the 'grip' on the bullet by the brass the same from round to round. This is done along with other things such as primer pocket prep (et al) to make ignition ( and thus pressure rates and limits) consistent.

Another application is to ensure reliable function when the chamber has been machined to promote very tight tolerance between chamber and cartridge dimentions, again with the goal of maximizing shot conformity.

Another use might be in making sure that when converting brass of one cartridge for use in constructing another. A simple example would be converting a mixed lot of .223/5.56 brass to 300 Blackout. When converting, part of the wall of the parent brass becomes the neck of the new case. In this instance, the wall thickness of the case varies from manufacturer to manufacturer. Some of them are not ideal after forming the new case for function, from feeding into the chamber, or from too much neck tension which can cause pressure spikes which can lead to less consistent muzzle velocity.

Generally speaking, if you are not into a shooting discipline where extreme brass prep is the norm, don't worry about it. As an example, a few years ago, Precision Rifle Blog .com posted a survey of the top PRS shooters and what they did for reloading prep. The majority of them did not concern themselves with neck turning/using a mandrel when reloading.

I think if you are chasing low ESD, there are more effective ways to try before getting into using a mandrel and neck turning brass.

If you are interested in getting into the weeds about reloading or just like making things with precision without worrying about utility, I say read up on it and go for it!
 
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Thanks for the well put together overview.
I did do a little light reading on the subject of neck turning just to satisfy my own curiosity. Suffice it to say, I won't be going down that road any time soon! Real deep in the weeds, micro tuning type stuff.
 
Thanks for the well put together overview.
I did do a little light reading on the subject of neck turning just to satisfy my own curiosity. Suffice it to say, I won't be going down that road any time soon! Real deep in the weeds, micro tuning type stuff.
This guy has never turned a neck ever.

 
The basics to consistent shooting are
1) Load development/Velocity node. Think barrel time & harmonics.
2) Case volume/pressure consistency (includes sizing, necks, seating pressure)
3) Seating Depth

Neck thickness is measured with a ball micrometer. (~ $40-$240)
Even quality brass varies in neck thickness by a slight amount which allows neck tension on bullets to vary from case to case which leads to varying scores.

Some top shooters will turn necks on new brass so it is a consistent thickness. (~$100-$800)
For some that is .014

Neck sizing pushes any high spots into the neck. Expanding the neck attempts to correct that.

Setting neck tension starts with knowing your finished cartridge's diameter with a bullet seated. If the neck thickness varies, any measurement you take will vary based on where you take it. Turning necks makes that more consistent.

Take a 6mm cartridge, the bullet diameter is .243 and you add the brass thickness on each side to get the final number. If your brass is turned to .014, you add .028 to .243 and end up with .271

If you are using bushing dies, you could select a .269 bushing and have .002 of neck tension.

If the neck thickness varies, the tension on the bullet varies. In addition to turning, you can expand to push those inconsistencies to the outside of the neck where they won't affect the bullet.

Selecting an mandrel to expand should take into account brass will spring back.
Say you expand with a .242 mandrel, you have reduced your neck tension to .001 but the brass will spring back
.0005 so you end up with .0015 of neck tension.

Calculating neck tension isn't the end goal, making your seating pressure consistent is. The steps to prep necks are designed to get you to the same seating pressure when seating bullets. If you have bullets that are hard to seat mixed with bullets that seat much easier, these rounds will have varying pressure and varying scores.

Measuring seating pressure can be be done with an arbor press with a force pack gauge (~$150-$250).
Here you are looking for consistency. The rounds that seat with the same pressure tend to shoot the same. Now you need in line seating dies to go with your arbor press. ($65-$500 for custom reamed dies)

Prior to seating you can precheck the necks of your brass with pin gauges (~$5 each).
Sorting here will make sorting during seating much easier.
Just doing this once can be eye opening with respect to how necks can vary.
Here is where the benefits of annealing your brass come in.
Regularly annealed brass will have more consistent properties like spring back. (~ $500-$1500)

Ok, so add up the cost of the tools and the time to prep brass and decide if it is worth it.
Many competitors feel it is.
If you are loading for varmints, you probably should spend the time loading more ammo

Want to cheat and shoot accurately without all the gear? You can shoot quality virgin brass that has been expanded prior to seating and get very close.

The basics to consistent shooting are
1) Load development/Velocity node. Think barrel time & harmonics.
2) Case volume/pressure consistency (includes sizing, necks, seating pressure and bullet sorting)
--- Expanded New Brass checks this box
3) Seating Depth/Bullet preparation
 
Wow, thank you whiskey. Very informative.
So, what I got from that is, consistent neck thickness= consistent neck tension = consistent seating pressure, which all adds up to a more consistent release of the bullet from the case. End result being a more precise round.

Damn! Now I want to go measure all my case necks.
 
The replies above have summed it up pretty well.

Depends on how far down the rabbit hole you want to venture. Measurement Tools provide information. What you do with that information, or deciding if it's worth knowing is up to the individual and how much time and energy they want to put into making the perfect cartridge.

If the shooter or shooting platform (gun) aren't capable of realizing any benefit from the tools, then the time and effort might be better spent upgrading equipment or putting more time behind the trigger.

It's a very time intensive endeavor to try to make a perfect cartridge. However the results are worth the time for many.

I'd suggest picking up a book or two on precision reloading if this type of thing interests you.

Mic McPherson and Glen Zediker are two authors I've relied on for such information for quite some time.
 
If you use one, what's the purpose?
Are you comparing different cases against eachother, relative thickness of a single neck, or what?
Checking neck Thickness and neck runout. Using this information in order to segregate or determine if further processing needs to take place.
 

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