neck tension

[Bart B.]

Case "neck tension" is typically expressed in thousandths of an inch.

It takes much more force to push a 30 caliber bullet out of a case with .020 inch neck wall thickness than one with .010 inch" neck wall thickness and both have .001 inch "neck tension."

There's sound reasoning behind the force needed to push or pull bullets out of case necks being the industry standard. It's called bullet pull force.

[BJung]

My "guess" is that neck tension is determined by the resized case neck with a certain case thickness as mentioned above in relation to the bullet diameter. One remedy is to take a mandrel and size the inside diameter to a consistent width and turn off the excess so neck thickness is consistent so the future outside neck resizing of the brass is consistent. Another method might be to take a neck sizing mandrel used by cast bullet makers and enlarge the inside diameter of the case neck so your resistance will be the same. The only caveat would be that the thicker necks still might provide more resistance because of the extra metal but I think this is minimal.

[hounddawg]

I messed around using a load cell and a arbor press a couple of years back and made a few observations about surface friction and neck tension.

- When starting to seat a bullet "x" amount of pressure was needed to start the bullet moving into the case.

- Once initial surface friction was overcame pressure dropped but you could cause spikes by simply increasing the pressure on the arbor handle which simply cause th ebullet to enter the casing faster

- the same would apply to pulling bullets. Initial pressure to overcome surface friction and the harder the pull the faster the bullet would exit

- using lubes decreased the seating tensions and made the seating pressure required more consistent but had little or no effect on velocity consistency or on paper

to get a true measurement would require a electronic control to ensure that the pressure would be even otherwise any measurement is iffy at best. rather than run down a rabbit hole of guesses it's better to just go shoot and see what works on paper

Bryan Litz did a study where he found that what we normally call a "neck tension" of .003 - 004 provides the most consistent velocities. My own studies which were casual at best showed me that lubing the neck or bullet using dry and liquid lubes had no effect at all on velocities.

I found by having a consistent neck thickness and ensuring my cases were sized to give a firm hold on the bullet without damaging the jacket gave me the consistent grouping I was wanting. I use .004 neck tension on my BR rifle and at 100 yards I get .1 to .2 MOA and on my F class rifles less than .75 MOA verticals (20 round groups) out to 800 yards using .003 on both the .260 Rem and the 6 BR . I wish that pesky wind was as easy to beat as the reloading was.

[Bart B.]

Originally Posted by hounddawg

Bryan Litz did a study where he found that what we normally call a "neck tension" of .003 - 004 provides the most consistent velocities.

Without the case neck thickness and hardness as well as contact area on the bullet, a given interference fit dimension can have a wide range of force required to push bullets out.

3 to 4 thousandths seems several times as much. You'll need the die neck diameter to be 5 to 6 thousandths smaller than the cartridge neck diameter to do that. I think a lot of copper will be scraped off bullets doing this.

2 thousandths smaller die neck is typical these days. And gives about 1 thousandth interference fit.

[BJung]

can you rephrase all of that? Bart, are you saying 3-4 thousands case neck thickness as the difference from the outside case neck diameter to the inside case neck diameter?

Would another approach be to seat and pull a bullet thus using the bullet as a mandrel. Then, take the inside diameter and turn the neck to a consistent thickness?

[hounddawg]

like I said Bart to me the proof is on the target, theorize to your hearts content but I am getting results on the targets so I must be doing something right. Funny thing is my reloading procedure is dead on simple and my tools are bargain basement with the exception of my sizing dies (Redding or Whidden bushing dies) which I consider the most important part of the process

the way I check my neck tension is simple and fool proof. Size a case , measure the OD of neck, seat a bullet, remeasure. That is my neck tension if there is less than .003 expansion on the second measurement I go down another .001 on the bushing until I get a minimum of .003 difference. Ever see a bullet that has been sent down a barrel ? That rifling gouges bullets a plenty. That is why I like to buy use blems. I don't sweat the small stuff. When I miss the ten ring 99% of the time it is me not judging wind or mirage right, not the bullet or rifle.

[Unclenick]

The problem is that everyone is actually describing interference fit with the neck, not tension. As Bart points out, if the brass is thicker, the pressure needed to stretch the brass the same amount is greater. That pressure, multiplied by the contact area between the bullet and neck and by the coefficient of friction between the two will give you bullet pull force.

[Bart B.]

Originally Posted by burbank_jung

Bart, are you saying 3-4 thousands case neck thickness as the difference from the outside case neck diameter to the inside case neck diameter?

Would another approach be to seat and pull a bullet thus using the bullet as a mandrel. Then, take the inside diameter and turn the neck to a consistent thickness?

Answer to both questions is No.

Interference fit is the difference between resized case neck inside diameter and the bullet's diameter.

Use a neck turning tool with a steel pilot mandrel to uniform neck wall thickness uniformly:

https://www.midwayusa.com/case-neck-...s/br?cid=10455

You may need to resize the case neck after uniforming its thickness.

[Nathan]

Bryan Litz did a study where he found that what we normally call a "neck tension" of .003 - 004 provides the most consistent velocities. My own studies which were casual at best showed me that lubing the neck or bullet using dry and liquid lubes had no effect at all on velocities.

Wowzers! I would destroy billets with that kind of neck tension. At 0.003”, bullets had a firm ring in them. In my testing less was more generally and I think I arrived at 0.0015” approximately as perfect with graphite lube in the neck. Necks annealed. I need more testing to proclaim any value from these numbers, but they did work once for me.

[Bart B.]

Note that if the case mouth inside edge is too sharp, it'll peel off jacket strips unbalancing the bullet as it's seated.

[hounddawg]

@UncleNick - that's great but in the real world most of us do not have the training or equipment to determine real neck tension. For those who want to then clean the brass to bare metal, measure the thickness to .0001 and here is the math

https://study.com/academy/lesson/ela...surements.html

so instead I use the interference fit and call it neck tension, it uses tools that every reloader haas on the bench all my range buddies know what I am talking about when I use the term and I can judge the effects of changes with a target and a chronograph with the chrono being optional

@ Nathan

Litz, Bryan. Modern Advancements in Long Range Shooting Vol. 2: Volume II . Applied Ballistics, LLC.

in table 6.5 Litz calls .0005 interference as very light, .001 as light, .002 as nominal .003 as relatively heavy .004 as heavy and .005 as very heavy and likely at that point to damage bullets. In his tests he found .003 provided the most consistent velocities in .223 and .243 and with .308 it did not matter. I choose to use .003. With cases where some or all of the brass has been work hardened enough to display springback after sizing I go with .004 to allow for the springback so the brass ends up at what most shooters and myself call .003 to .004 neck tension. I chamfer the mouths, do not use any lube and never had any copper shave off yet.

[Bart B.]

I think some folks forgot a zero: .0005 inch instead of .005 inch.

You can make a tool to measure bullet pull force with a hand scale and collet style bullet puller.

Remove die from your press then put a cartridge in the shellholder, top it out, tighten collet on bullet, attach the scale then pull the bullet reading the scale. Do several to get average.

Military 7.62 NATO match ammo bullet pull spec is 20 pounds minimum, 60 pounds minimum for most service ammo.

[hounddawg]

ok typo corrected and if you want to jump through hoops for some number that is probably impossible to duplicate that's your business. I'll keep using the definition that most the reloaders out there use and that is easily duplicated. If I tell you my neck thickness is .014 thick and I use a .289 bushing you can duplicate my neck tension by turning your necks to .014 and using a .289 bushing. Simple and effective

[Bart B.]

For what it's worth, most military rifle cases are made with neck inside diameter about a thousandth inch or so larger than bullet diameter. Would they have a minus neck tension?

If several people use .003 inch "neck tension" across different lots and dimensions of case necks, bullet pull forces will vary from lot to lot.

[hounddawg]

it's really pretty simple - bushing diameter minus neck wall thickness multiplied by two and subtracted.

example .289- (.014 x 2) = .261, bullet OD is .264 or .003 larger. or
290 - (.014 x 2) =.262 bullet = .264 or .002 larger

You can change bushings or turn neck brass to adjust to get the tension you prefer.

or https://forum.accurateshooter.com/th...-dies.4019066/

or use a Lee collet die and tune it like John Keilly does

https://forum.accurateshooter.com/th...#post-36644901

or just get a non bushing die, size the case then seat the bullet and shoot

Remove die from your press then put a cartridge in the shellholder, top it out, tighten collet on bullet, attach the scale then pull the bullet reading the scale. Do several to get average.

images of bowling balls on ramps, cats in bird cages and fans knocking over books in a domino effect comes to mind - Rube Goldberg would be proud

[Bart B.]

Here's a good article on bullet pull force:

https://www.google.com/url?sa=t&sour...ZfC5mWDmbPn_Wx

[hounddawg]

thanks for the link Bart but why should I care about bullet pull values? To me the hobby is about getting my groups consistent and as small as possible on paper, I have no need to know what the pull values are. I simply need to know how what the effect is my targets and be able to adjust

I can judge my velocity data and consistency measured with a chrono, Group size and consistency is measured with my calipers, and I can adjust neck tension to tune those numbers by changing out a $15 dollar bushing or turning the neck brass down .0005 or .001 or whatever value I want. That's all I need.

A tensile tester would just be a $3K paperweight spitting out what to me are meaningless numbers. At the least designing and building of a cat in a birdcage Rube Goldberg pull force tester would be entertaining

[Bart B.]

Originally Posted by hounddawg

thanks for the link Bart but why should I care about bullet pull values?

Bullet pull forces determine the pressure needed to push bullets into the rifling. The more uniform they are, the more uniform velocity is.

[old roper]

You can get one of these.

https://www.precisionreloading.com/c...=KM&i=ARBLPDRH

[Bart B.]

Originally Posted by old roper

You can get one of these.

https://www.precisionreloading.com/c...=KM&i=ARBLPDRH

Seating force is usually different (higher) than bullet pull (or push) force needed. The case neck diameter gets expanded bigger.

Especially when the case mouth inside edge scrapes off jacket material; more force is needed to push bullets into case necks.

[hounddawg]

Bullet pull forces determine the pressure needed to push bullets into the rifling. The more uniform they are, the more uniform velocity is.

In my own shooting I experimented with various neck lubes and while they did provide a "smoother " seating feel I saw no difference in velocity consistency or point of impact on target. My five round SD's are consistently single digits, I can hold .75 MOA or less group heights consistently out to 800 doing nothing to my case necks except trimming, chamfering and sizing them tight.

Now on the hypothetical side would altering the surface friction with lubes and polishing make a difference in a 1000 yard long hypothetical wind free test tunnel shot with perfect bullets and perfect loads shot from a machine rest. Possibly. However in the real world the ability to read the wind signs and or shoot through mirage that makes your scope look like someone has smeared vaseline on the lens is what makes the difference when the target is scored.

You guys enjoy your back of the gun shop discussions on how many angels can balance on a kernal of Varget. Me I will be out in the real world spending my time and money practicing my shooting skills while others search for the magic pixie dust

[old roper]

Bart, you control inside/outside neck dia by bushing or expander If the start is the same won't bullet pull be the same? You can always do the what if.

You have better way you can always bet me.

[Bart B.]

Originally Posted by old roper

Bart, you control inside/outside neck dia by bushing or expander If the start is the same won't bullet pull be the same?

No.

Measure seating force on 10 cases, then 24 hours later measure the pulling force noting the difference.

Repeat with 10 other cases waiting 4 days.

[BJung]

Something to consider. I once was surfing the web for long-term ammo storage and natural welding of the bullet to the case. A comment mentioned was that a clean inside neck and bullet without contaminants was the best procedure while coating the bullet didn't help. Maybe sealing the mouth by the bullet to keep moisture from the case and bullet like old military surplus bullets helps long-term storage but doesn't apply here. In short, polish and clean the inside of your case mouth and bullets.

[hounddawg]

In short, polish and clean the inside of your case mouth and bullets.

No offense but what you want is a dry, but not clean contact. I believe a coating of dry graphite would be perfect. When you have two solid surfaces in direct contact, adhesion can occur. If two solid surfaces are clean there will be adhesion and bonding of one solid to another. Oils and anything that dries out over time is another no no. Oxidation of the brass and the copper can mingle and result in cohesion due to molecular bonding of the oxides.

I believe the military used lacquer or some type of sealant around the junction for long term storage, Bart can probably clarify that.