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SIMPLE question on neck tension


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I have read and view videos on bullet neck tension until I was dizzy. ( doesn't take much )
Am I over simplifying this or is this is what you need to know.
I understand that a lot of variables go into this.
Wall thickness, concentricity, straight parallel and all, but
The basic drawing I submitted, YES or NO!
Using .001 inch bullet tension as an example.
Any and all comments would be appreciated.
Thank You


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  • 1 month later...

Your math on the illustration isn't accurate but that's probably irrelevant to the conversation. Unless you are shooting bench rest you need neck tension. I use .003" measured by the difference of a resized case neck prior to seating a bullet, compared to the neck diameter of the finished round. You need .003" for semi-auto, .002" for a bolt action.


You can increase neck tension by spin polishing the expander button in a power drill. I use 400/600 grit wet/dry paper to reduce diameter. 1000/1500 for final polishing. Go slow. Remove only .0005" to no more than .001" of material at a time and retest by resizing some brass. .306" expander ball will usually net an internal diameter of .3055" to .305" because the neck will spring back a little after the ball passes through.


You can buy pin gages and I have an assortment for the different calibers I load. A set of .3045", .3050", .3055", .3060", .3065" .3070" or simple .001" increments would allow you to measure actual internal diameter of any resized case (.30 caliber).


.305" internal neck diameter works well for me across all of my bolt actions and semi-auto .308's. No crimp is needed or wanted.

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For accuracy, never use extender buttons....ever..they will create donuts in your brass....when you fire a case, it will straighten to conform to your chamber. Use a full length neck bushing sizing die that allows you to bump shoulder of case back about .002 and choose a neck diameter bushing and expander mandrel that proves most accurate for you load. After sizing you case(use lube for smooth resizing), then get a couple extended mandrel’s and use them to control your neck tension. 21st century makes these in 1/2 thousand sizes. This is important..your extender mandrel is what controls your neck tension, not your neck bushing die.


My routine- I pull fired straight from box and anneal with my AMP. I then full length size,  bumping shoulders and sizing neck( I remove extender button from die and use a neck sizing bushing that is .002 under diameter of the expended mandrel I’m using). Now I use the extender mandrel to open up every case neck. Once this is done, I tumble the case in a walnut media....bare in mind that I’ve used plenty of imperial wax on cases while sizing. When you tumble the brass in the media, the wax off the case will collect on the media lightly-this will actually lightly lube/clean the inside the neck cases while clean the outside of cases. Under sizing the necks and then using the extender mandrel, along with annealing and tumbling your brass which cleans and lubes inside the necks, makes every cases neck tension as close to one another as possible. You control neck tension with neck sizing bushing and extender mandrel..and every gun is different on what it likes for neck tension...let the gun tell you what it likes...I hardly ever run over .0015 tension on any target rifle that I own..

My no turn neck 284 for example. A loaded round neck measures .315 with a Berger 184. You should check this anytime you change bullets types or weights.  I use a .319/.320 chamber reamer when chambering which gives me .004-.005 neck clearance total. I use a .311 neck bushing in my full length sizing die, then follow with .313 size extender mandrel. My case necks now measure .314 before seating bullets. This gives me exactly .001 neck tension on ever single case assuming every case is annealed the same and every cases neck thickness measures the same all the way around. Turning necks to control this is another subject.


I shoot very little benchrest...most long range F-Class. I average 3000-5000 rounds a year. I hope I made some sense above..

Edited by falconpilot
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I am going to answer your question simply:  Yes..  Measure the ID of your case necks and it will be smaller than your bullet.  How ever smaller is your common and usable neck tension. 

try shoot for a .306 id case mouth for a .308 bullet and see what happens to include checking sd and es numbers.


----continue for more technical if you really want to but that was the short answer to your question----


Yes, other factors do effect neck tension; however with any given, specific stick of brass and any given specific bullet, a smaller id case mouth = more tension.  Change the case and make it harder or softer or a bigger or smaller bullet and yes tension changes.


If you size a given stick of brass in what ever die you have, it should be .003-.001 smaller than the bullet.  If its .004 or more smaller, that's too much and you will likely damage your bullet or shave it.  If you have zero neck tension or a .308 hole and a .308 bullet, you wold be able to press the bullet by hand, a .308 bullet in a .309 hole would fall out.


bolt guns can go .001-.002 most of the time, most semi autos do well with .002-.003.  


Some dies are so bad that they will give you .005+, i used to have a .223 die that resulted in an id of .219; bullets were damaged and that did did not work.


Bushing dies allow you to adjust NT without turning brass.


You can increase NT in a common die by reducing the button but a given measurement will only be constant with a specific brand or lot of brass; it would not be the same with LC and Lapua.


I have never worried about doughnuts and you dont always get them. 



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  • 2 weeks later...
  • 3 months later...

Some very good info above. Regardless how you get there, gage pins are the only way to accurately measure case neck ID. As was also stated, your drawing was incorrect. .30 cal bullets measure .308".


So.................... .002/.003 neck tension works just fine for a bolt action with NO crimp.


The rule of thumb we use ( look up the origin of 'rule of thumb' it's interesting) is target ammo never gets a crimp. Hunting ammo may be cycled in and out of your rifle many many times without being fired. Crimp hunting loads, the Lee factory crimp die is a good choice. 

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I think first one needs to know his desired neck tension. As mentioned in previous post .001-.003 is most common. I usually aim for .002. There are 2 ways I acquire that goal. One is with bushing dies and one is with expander mandrels. To select the proper bushing you need to know the outside diameter of a loaded round.  That measurement is 2 times the neck wall thickness plus the diameter of the bullet. ie. Brass wall thickness is .014" for example and bullet is .308 ".    2 X .014  = .028    .028 + .308 = .336      .336" is outside diameter of a loaded round in that particular brass. Therefore a .334 neck bushing should give you the desired .002" neck tension. All this is related to brass spring back. If you anneal, which I highly recommend,  spring back will not be an issue. I NEVER use the expander balls as they pull UP through the case neck and can really negatively effect concentricity of the loaded round. Straight loaded rounds shoot straight. 


Another way is using the expander mandrel. My experience is this way, although an additional step, gives the LEAST run out to your loaded round. I either use a full length non bushing sizing die or a bushing die that sizes the necks down about .005. Come back with a mandrel that sizes down through the top of the neck expanding the whole inside diameter at once. Using the above measurement I would use a .306" mandrel in this situation for a desired neck tension of .002".


Brass spring back can be a bugger in all these procedures therefore careful measuring is needed when you first start sizing to confirm the results and or tweak the size bushing or mandrel. Controlling brass neck softness is a whole other conversation.


There is lots of information on rifle specific forums such as Accurate Shooter, Long Range Hunting Forum, Longrange Only, and Snipers Hide.  Good luck, research and ask lots of questions



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  • 1 year later...

YES.  The simple answer is you understand the basic concept of neck tension.  Like anything else regarding precision reloading there is never a simple answer.  I think I agree with most everything posted in this string, not sure I agree with fouling inside the case neck though but I am very particular regarding case cleaning.  I am an F-Open shooter currently campaigning a 284 Win running Lapua Brass.  I run a .315 chamber which means I have to turn necks to get the desired neck to chamber clearance, I try to have my completed cartridge measure .3115"  +/- .0004" at the neck.  As hard as I try to make them all exactly the same when measuring out to the ten thousandths ( .0001") , they will not be exactly the same.

How much neck tension is the right amount?  Many people will say “just enough”  meaning run as little as you can without having the bullet move after you seat it.  I am not sure I agree with that.  I have found my rifle likes a little neck tension.  Every barrel is different.  Different cartridges are different, the 284 likes more tension than the 6 Dasher.
I am surprised no one mentioned measuring neck tension when you seat the bullet.  By using an arbor press with a dial indicator you will both see and feel the amount of pressure it takes to seat the bullet.  That pressure equates to the amount of neck tension.  The more effort / pressure it takes to seat the bullet the more neck tension you have.  The dial gauge will quantify the reading for you.  
If you are not an F-Open competitor that obsesses over every detail involved with reloading I would recommend the following.  Fire form your brass.  Decap only and then clean it thoroughly.  I anneal after every firing but you can cut that back to every 3rd firing.  I use a whidden bushing die because it is one of the few that will both bump the shoulder and size the neck at the same time.  Take the decapping rod and expander ball out of the die.  I do not recommend using an expander ball on anything other than spray and pray ammo.  If you use a bushing that is .002” smaller than your completed cartridge you should be in the ball park.  If you want to obsess over neck tension you can get multiple size bushings and invest in a set of expander mandrels from 21st Century that come in .0005 increments and expand back up after you down size with the neck bushing and play with different combinations of the two.  All brass will have some “spring back”.  If I down size with a .309” neck bushing and then run a .2840” expander mandrel through it the inner dimension will not remain at .2840” it will spring back to something smaller than .2840”.  The proof will be in the dial indicator when you seat the bullet.
Help this helps.
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