Quantifying neck tension and bullet seating force.

I’ve been reading a lot about how neck tension affects velocities and therefore precision. We neck turn, anneal and spend a great deal of time preparing brass but how do we quantify / measure the actual neck tension on the bullet? One solution I found was to use an arbor press with the force measurement option then sort the rounds based on seating force. With that idea in mind I bought a press with the force measurement option along with a Wilson Micrometer arbour seating die. I then loaded 120 rounds of 6.5CM where the brass had been annealed before neck uniformed to 0.014" and neck sized to 0.002" under. I then took notes of the force required to initiate seating and was surprised at some of the variations. The least was 10lbs and the max was 90lbs but the majority were around the 30lbs mark.

Press - http://brtshooterssupply.com.au/products/km-arbor-press.html
Force Measurement Option - http://brtshooterssupply.com.au/products/low-force-pack-for-arbor-press-km.html
Dial Gauge - http://brtshooterssupply.com.au/products/km-dial-indicator-001.html
Wilson Die 6.5CM, S/S, VLD Stem - http://brtshooterssupply.com.au/products/wilson-stainless-micrometer-seater.html
Being a nerd with a background in electronics I wanted a better way to measure the force so I purchased a 200lb load cell and a Digital display. The joy of the display / controller is that I can set a low and high alarm so I don’t need to watch a dial gauge when seating. For example on the 6.5 I can set a low of 28lbs and high of 32lbs and reject any rounds that trip the alarm. The failures can then be assessed for either annealing again or a tighter collet to size the neck.
Display - https://www.tomtop.com/p-e3684.html
Load Cell - https://au.element14.com/sensor-solutions-te-connectivity/fx1901-0001-0200-l/compression-load-cell-200lb-5vdc/dp/2717359

Now to machine a small collar and platen for the cell before calibrating and more loading.

Why do I bother you ask, I’m bored and any excuse to merge shooting with technology is fun for me :slight_smile:


That is truly epic! Love it!

Looks good @Brett there are a lot of shooters esp in the US that swear by the method you are using. I think it is a pretty good indication, but am not totally convinced. It certainly give you the resultant force that is require to seat the bullet but once it is seated will that be the same force that is actually holding the bullet in place? Is it the same force that will be required to extract the bullet.

An option I have thought about but will never try. Is after the neck thickness sizing or uniforming to be more correct. Some device would be needed to place the bullet into the case under very little tension to none ideally then to use a device like the Lee Factory crimp die ie a sort of squeezing die that could then have the same force used to press the neck against the bullet.

Neck tension is an interesting topic. I look forward to seeing or hearing about your results. The results are so hard to quantify in shooting. I often seem to be chasing my tail going around in circles as getting consistent results over a long period of time seems nigh on impossible.

Looks like a fun project, Brett. I’m sure it will help shot for shot consistency.

I think we have touched on this before but have you experimented at all with the neck tension/loading techniques described in the Houston Warehouse Accuracy Project?

For those unfamiliar, here is a link to a sumary and a PDF download of the original article about the Houston Warehouse Accuracy Project: https://precisionrifleblog.com/2013/10/18/secrets-of-the-houston-warehouse-lessons-in-extreme-rifle-accuracy/

Also, Brett: I kind’a expected to see you in this vid the other day! :mask: :+1: :laughing:

@Gwion I was at KOT2M at Raton New Mexico when they filmed that series and met Ronnie and a few of the guys including Bryan Litz from AB. I actively avoid getting my face in pictures or on TV since my appearance in a serial killer re-enactment video so you won’t see me in this one. I do recall reading that article some years ago and they place a massive importance on neck tension being critical to precision. What did puzzle me was that they sort cases based on mass rather then capacity, I’d have thought capacity would be more important ???
The Test will be (when we can shoot again) that I’ll take 5 rounds with the lowest tension and 5 with the highest and run them over the chronograph then compare the results. Seating by “feel” doesn’t work for me cause it’s subjective and not easily measured. I will also load 2 lots of 5 rounds with the neck sized at 2 & 8 thou to see if a trend develops in velocity.
The bit that surprised me the most is the near 75lb variation from lightest to heaviest seating force considering all cases were prepared identically in 1 batch using the same tools and from the same lot of once fired Hornady factory ammo. Next investment I think will be an AMP Annealing machine just to eliminate any minor differences in annealing.

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:fearful: :pleading_face: :scream: :skull: :skull_and_crossbones: errr… re-enactment???

Totally agree that the article’s reference to adjusting tension ‘by feel’ is rather out of step with their otherwise meticulous adherence to measurable data analysis.

It will be interesting to see your data once you have a chance to get out on the range again!

From my understanding of the article, while neck tension was critical, it worked in conjunction with seating depth. Reloading gear being what it is, there is always some vagary with exactly how the bullet is seated and so their answer was to have the neck tension such that it just holds the bullet in place but can move back in the neck with light force. When the bullet is then seated long, it engages the lands of your rifling and seats itself to the same exact depth every sing shot. The fact that this all relied on precisely machined chambers and turned/reamed cartridge necks aside, this is my understanding of their findings. Not a technique for magazine fed rifles but certainly worth exploring for single round feed just to see the data. If you could determine how much force is needed to a/ move the bullet in the neck and b/ jam the bullet in the lands you could come up with an optimum neck tension to hold the bullet and consistently engage the lands for every round. Just throwing ideas around…


I think you may find this useful, quick search, but I am fairly sure there is a library somewhere on GitHub specific to this sensor. Or just calculate your own output…

Add cheap breadboard kit, to avoid making things permanent (while testing)… And I think you could splice in, between the display and the sensor, to add RPi in the middle, to actually log data to a file. That would be the bit that requires the most amount of tinkering.

Then, with something like inotify (IN_MODIFY event), get the updates as they happen, parse (trivial) and save to MySQL or something similar. Add MySQL as a data source for Excel, bam! You are getting real time data into a document on your workstation, as you work, directly form the device :slight_smile:

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While i don’t reload, the OCD in me is just loving the level you’re taking this to @Brett.

I’m under the impression their reasoning was that there is a direct correlation between case mass & capacity… will have to re-read it myself now!

I’ve measured case mass and capacity and there’s no reliable correlation as I’ve had cases that weigh the same but varied a significant amount in capacity. Why is this so? Glad you asked cause I scratched my head on it as well. Think of how the case is made, it starts life as a sheet of brass then punched into cups before being drawn / annealed several times into its final shape. During the forming process the brass can be compressed different amounts (forging essentially), particularly the base which will change the capacity without a change in mass.
Actual case capacity in gr/H2O is the most reliable imho as it dictates the amount of “free” space or “charge compression” in the cartridge which is critical for ignition / burn / pressure consistency. As I chant 1911 times quietly to the god of all things shooting John Moses Browning, “Consistency is the key to precision” :slight_smile:

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Mate that is taking precision to the next level. Great read mate and looks like a great project. Unfortunately I don’t have the brains for that and will just trust that my basic reloading processes are satisfactory.

@Brett I dont have an AMP but I think I have its home made cousin it uses induction and is timed to less than a 10th of a second. Happy to anneal some brass for you if you want to extend the tests. I am really interested in seeing your results.
I am trying to determine if I can see a difference using a Redding bushing die or the Lee collet die.

@sungazer The AMP machine has been on the “List” for ages and I’ve got the $ saved for a new pistol but might put that on hold for a month and get the AMP next week. If the AMP isn’t security sealed I might open it up and see what hardware they use to analyse the case during AZTEC discovery mode. I’m guessing some type of IR temp probes to measure the heat of the neck / shoulder and how long it takes to get there. I use a Forster bushing bump die to size the brass with a collet -0.002" from seated size and a Wilson S/S Micrometer Arbor seating die cause I consider them the gold standard in seating dies due to their chamber based design. Loading 120 rounds and having <0.001" runout at the ogive saves me a bucket load of time by not having to adjust concentricity.
Once the range opens or I can get bush I’ll run them over the LabRadar and see if the data follows a trend consistent with the seating force. If I get time I’ll glue the Pressure Trace II sensor on the barrel so I can graph the chamber pressure as well.

The AZTEC mode works by monitoring the current to the Ferrite core. When the brass actually starts to change structure bright red to melting the current decreases quite a bit. So they monitor that current and the AZTEC mode destroys a case finding out how long that actually takes and then they have some sort of formula to work back from that point to a correct time to anneal that case.

I think the actually take the temp up a little hotter than we have been led to believe as in no hint of change to the colour should occur.

I have a heap of pictures of the unit taken apart if your interested. AMP belive it or not have been very helpful to the DIY induction community. There reasoning is that there will only be a very small number of people that go to the trouble to build a machine themselves but the more people sprooking Induction the better it will be for them.

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