Bushing vs. Expander Mandrel For Obtaining Neck Tension

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Red Wolf

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Aug 8, 2019
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#1
So I know "neck tension" can be a hot debate, but I was wondering if anyone could shed some light on using a bushing vs. an expander mandrel for obtaining a desired amount of neck tension. Right now I run all of my new brass threw a 0.263" expander mandrel (shooting 6.5 creedmoor) to obtain 0.001" neck tension and my gun seems to really like that. However, once the brass is fired I use a full length resizing die with a 0.290 bushing since my total neck OD of my loaded round is 0.291" to obtain 0.001" of neck tension. I don't use a neck expander inside of my bushing die.

Does it make a difference whether you're using a bushing or an expander mandrel to size necks? Obviously the bushing sizes the neck from the outside while the expander mandrel sizes the neck from the inside. Is there any advantage to one or the other? Is there an advantage or disadvantage to using a bushing and then an expander mandrel after that?
 
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#2
Red Wolf said:
So I know "neck tension" can be a hot debate, but I was wondering if anyone could shed some light on using a bushing vs. an expander mandrel for obtaining a desired amount of neck tension. Right now I run all of my new brass threw a 0.263" expander mandrel (shooting 6.5 creedmoor) to obtain 0.001" neck tension and my gun seems to really like that. However, once the brass is fired I use a full length resizing die with a 0.290 bushing since my total neck OD of my loaded round is 0.291" to obtain 0.001" of neck tension. I don't use a neck expander inside of my bushing die.

Does it make a difference whether you're using a bushing or an expander mandrel to size necks? Obviously the bushing sizes the neck from the outside while the expander mandrel sizes the neck from the inside. Is there any advantage to one or the other? Is there an advantage or disadvantage to using a bushing and then an expander mandrel after that?
Click to expand...

I use a mandrel for all sizing. If neck.sizing, I use a bushing that gives me .004 under loaded round then run it into my mandrel that gives me my final .002 neck tension. I do the sams with FL sizing dies when bumping shoulders.
 
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#4
Oldmauser said:
@padom when you FL size the brass, do you pull out the pin out or do you FL size without any modifications to the sizing die?
Click to expand...
I do both. Some, mainly Redding type s, I remove the expander ball off the rod and just use the decapping pin retainer which won’t do any sizing to the neck.
Others I pull the entire expander assembly out and run it as a stripped die.

And some I’m lazy so I just leave the expander ball in and then I will run a mandrel afterwards just for the hell of it.
 
#7
Oldmauser said:
why?
Click to expand...
You’re doing it twice otherwise, and the ball is often viewed as inferior to a mandrel as it’s a “pulling” operation compared to a “pushing”.

I know quite a few shooters that size down with a bushing to 2-3thou under final neck tension size and then finish with a mandrel. My thought and theirs (as I do this too) is that with a mandrel, you’re pushing all the inconsistencies in the brass to the outside, rather than keeping any inconsistencies that may not be a perfect circle on the inside diameter as a bushing will do.

The other reason I size with a mandrel after downsizing with a bushing is that it allows me to account for differences in brass. I don’t know because I haven’t measured every brass in my lots, but I have to believe there is at least SOME variation, both in overall thickness, and thickness from one side of the neck to the other. By downsizing a few thou before opening it back up, I have tricked myself into believing that I have accounted for all these inconsistencies by sizing the ID of the case which, in the end, is what matters.

So do I know all of this for certain? No. But am I more confident in my loads because of it? Sometimes, maybe, no, yes.
 
#9
MtnCreek said:
I believe an expander ball is more likely to cause runout than a mandrel. I have not tested this.
An expander ball can cause inconsistent cartridge headspace. A mandrel doesn't.
Click to expand...
Yeah if you think about having a completely coplanar circumference on the bottom of your neck at the beginning of the shoulder, there’s absolutely no way to tell or to verify, so when that expander ball comes back up through the case, you have no way to be sure that the ball is making contact all the way around the circumference of the neck base, so when it pulls up, it can yank to the side. I imagine this is somewhat pronounced with cases that have the beginning stages of donuts.
 
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#10
I went to the mandrel by accident.

The problem I was trying to solve was inconsistent sizing due to the sizing button pulling the case back out after sizing.

My thinking was the case is stronger being pushed than pulled. The mandrel solved that problem.

In thinking about mandrel versus busing, my original logic would still lead me to the mandrel.

There will be irregularities in brass thickness in the neck. Do you want those irregularities on the inside (bushing) or outside (mandrel)?

Except for bulk training ammunition, I use a mandrel on all brass intended for serious purposes.

Works for me.
 
#13
If using a FL sizing die with a mandrel, I remove the entire decapping rod from the die. I use a Mighty Armory Blue decapping die in station 1 on my 550c to deprime, then the shoulder is bumped and neck squeezed down with the FL sizing die (decapping rod removed) in Station 2 then the mandrel in station 3 for final neck tension.
 
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#17
Oldmauser said:
I have a regular Forster FL sizing die. If I remove the rod from the die, would it still bump the shoulders?
Click to expand...

For guys not familar with dies work or havent been reloading long enough to know what these different dies do and how they work, I always tell those people to buy and use a FL sizing die. It makes excellent ammo.

Your rod has nothing to do with bumping shoulders......
 
#18
padom. i would really like to try your method of mandrel sizing the neck, how do you determine what size mandrel to buy. i do have have sinclair neck turning mandrels in different calibers dont know if they will work. i do have a lary willis shoulder bump gauge what do you bump the sholders to or does it vary.
 
#20
afv338 said:
padom. i would really like to try your method of mandrel sizing the neck, how do you determine what size mandrel to buy. i do have have sinclair neck turning mandrels in different calibers dont know if they will work. i do have a lary willis shoulder bump gauge what do you bump the sholders to or does it vary.
Click to expand...

I have 21st Century TiN "Turning" Mandrels from before 21st Century started selling different sized mandrels in .001 increments. So, their "Turning" mandrels give .002" neck tension.... Their Expander Mandrels give .001" neck tension. Sinclairs might be the same but easy to test, just FL size a case minus the rod, measure the case neck, then run it through the Sinclair Turning mandrel, measure the case neck again, then load a bullet and measure again.


(This is just an example of how to measure, numbers are made up)
Example:

Case Neck Size after FL sized WITHOUT expander rod - 0.260"
Case Neck Size after run through mandrel - 0.262"
Loaded Round Neck Diameter - 0.264"
This would mean you are getting 0.002" final neck tension

For my precision bolt guns I bump the shoulders .001-.002.... For AR's, I bump my shoulders .003"....
 
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#21
Oldmauser said:
Ok, what would be the consequence of taking the rod out? I assume the neck will be untouched, the shoulders will be bumped.
Click to expand...

No, that is not the case... a FL sizing die does the following things....

1. Deprimes the case
2. It pushes the shoulder back (amount based off how far you screw die down)
3. It squeezes the neck down as well
4. It opens the neck back up on the up stroke...


If you remove the rod you will not be depriming, and you will not be opening the neck back up (which you dont want to do if you are using a Mandrel to do that step.).

Just the Fl sizing die body setup properly will bump the shoulder back to whatever you set it to bump....and it will squeeze the neck down..
 
#23
The neck will be sized, probably a good bit more than required. The shoulder will be pushed back, the amount depending on how far you screw the die in.

iu
 
#24
Im loading for 6BRA and wanted to go with the mandrel approach and widden FL bushing die. Ending up a 2000 necktension.
I'm Sizing fireformet Lapua brass. My conclusion is that the springback in the brass makes it a very hard calculation.
There is difference in neck tension if I run the expander mandril in 1, 2 or 3 times.
Right now I damage the nose of the 105 hornady hpbts when seating them.

So my next step is to go one or two mandril sizes up, than the online explanations. I run a T24 now. So I'll go up 2. Run it into the neck once and then hopefully achieving that perfect neck tension every time.

Perfect ES/SD can for sure be found without. But I would like to achive that over the entire life of the brass. Its my belief that the mandril approach will make it more consistent. But once I have a good setup. I'll test 30 just with bushing and 30 with the mandril. Then let the numbers talk.

One question that comes into mind, is. Do this approach work the brass twice. So it actually work hardens faster. And needs annealing more often.
 
#25
MtnCreek said:
The neck will be sized, probably a good bit more than required.
Click to expand...


This is a whole different issue I chose not to bring up as not to confuse these guys even more.... But yes, commercial FL sizing dies like Forster, Redding, RCBS, etc. ALL squeeze the neck down WAY more than necessary and this is the very reason a lot of us have "honed" FL sizing dies... Or why guys use a neck bushing to squeeze the neck down then a mandrel. That way they can control how much the neck is squeezed down. I squeeze my necks down .004" under loaded neck diameter, that way my mandrel is only opening the neck back up .002"...leaving a final neck tension of .002... This works the brass the least.

People ask well why do FL sizing dies squeeze the neck down so much... Because these mass produced FL sizing dies are made to be able to size ALL the different brass on the market in that given caliber. And they all have varying neck thickness. Say you have a .308win and you bought a Forster FL sizing die. There are so many different .308win brass makers out there with a wide varying neck thickness, these dies are made to ensure they squeeze all these different brass cases down properly.... But if youre using 1 specific type of brass say Lapua for your custom 308win bolt gun, that die is not ideal for that specific setup if thats ALL youre going to use... I didnt say it wont work, its just not sizing the case minimally.
 
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#26
Bronden said:
One question that comes into mind, is. Do this approach work the brass twice. So it actually work hardens faster. And needs annealing more often.
Click to expand...

To answer your questions above, no it works brass less if your downsizing minimally and bumping the shoulder minimally then only opening back up a few .001's with a mandrel... Im a visual learner, so I tell guys asking these questions to go do some measuring for yourself so you can see why I/we size this way...

Go take your FL sizing die, set it up in your press to bump the shoulder back .002 (I assume you guys know how to set this up).. Now take a piece of fired brass in your rifle that hasnt been sized/touched. Measure that neck and write that number down. Thats your fired case neck size....

Now lube that piece of brass up, remove the decapping rod from your FL sizing die and size that piece of brass.. Measure the neck again. Write that number down.... Now subtract these 2 numbers... That will tell you how much your FL sizing die is squeezing that neck down before a) the ball on the decapping rod rips the neck back open or b) your mandrel opens the neck back up. Its going to be a huge difference if using commercial, non-custom or non-bushing dies. This is exactly why we use bushings or honed dies to squeeze necks down minimally .004 - .006 under final loaded neck diameter. This works that brass the least...

Ive never had the issue youre having with many years of using FL honed and bushing dies in conjunction with 21st Century TiN Turning mandrels.... Something isnt right. I always squeeze my necks down .004 under loaded when bumping the shoulder back, whether thats with my honed Forster FL dies or a FL Bushing die... I then run them in the 21st Century Turning mandrel for exactly .002" neck tension every time.... 6br, 308win, 223rem, 6.5cm, 6.5x47L, 220TB, 300blk, etc etc. I do this for all these calibers, same end result...

The springback calculation is for the squeeze down. I have found using .004" under loaded allows my specific mandrels to give me .002" final neck tension, single digit SD's across 25-35rd on the LabRadar and consistent seating...
 
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#28
Bronden said:
Im loading for 6BRA and wanted to go with the mandrel approach and widden FL bushing die. Ending up a 2000 necktension.
I'm Sizing fireformet Lapua brass. My conclusion is that the springback in the brass makes it a very hard calculation.
There is difference in neck tension if I run the expander mandril in 1, 2 or 3 times.
Right now I damage the nose of the 105 hornady hpbts when seating them.
Click to expand...

I have the same caliber, same approach, same die(!), same brass. What seating die are you using?

When LE Wilson came out with their 6 BRA seating die, I was all over it. I've been using an arbor press and LE Wilson dies to seat for a little while, and the process works really well. I've also started using graphite in the necks prior to using the mandrel, and some stays in for seating. This has resulted in very consistent seating force (which translates to neck tension, all other things being equal). Now, 9 out of 10 rounds fall within 10 lbs of seating force. I mark the non-conformers to use as sighting/barrel fouling rounds
 
#29
Rocketmandb said:
I have the same caliber, same approach, same die(!), same brass. What seating die are you using?

When LE Wilson came out with their 6 BRA seating die, I was all over it. I've been using an arbor press and LE Wilson dies to seat for a little while, and the process works really well. I've also started using graphite in the necks prior to using the mandrel, and some stays in for seating. This has resulted in very consistent seating force (which translates to neck tension, all other things being equal). Now, 9 out of 10 rounds fall within 10 lbs of seating force. I mark the non-conformers to use as sighting/barrel fouling rounds
Click to expand...

What are you using to measure the seating force?
 
#30
proff49 said:
What are you using to measure the seating force?
Click to expand...

I use the K&M arbor press for seating. You can get what they call a "force pack" which is essentially a constant force spring that goes on the press, along with a micrometer for measuring deflection. The "standard force pack" (which is what I have) has a spring that deflects .001" per pound of force applied to it. The micrometer measures the spring deflection, which measures the force applied.

I'll put up a video of it in action a little later.
 
#32
It should be mentioned that using the mandrel after sizing isn't a coarse adjustment, it's a fine adjustment. If it feels like your mandrel is doing a lot of work, then something is off and you're probably overworking your brass. This is why using a bushing or a custom honed FL sizing die is better than a "standard/normal" FL sizing die, because you can get them dialed in tandem to where they're working together without having your mandrel do too much and overworking the brass.

Annealing needs to be mentioned too, because it's a huge part of the whole deal: IMHO if you've got your bushing and mandrel sizes right, one should just kinda feel the mandrel doing its thing on annealed brass, and it's not working hard.
On brass that hasn't been annealed, the mandrel should feel like it's not really even doing anything 99% of the time... maybe you'll feel it on a few cases out of 100 fixing up some weird outliers that had wonky spring-back, but for the most part on non-annealed brass it should basically be doing nothing.

I mention the annealing thing because it basically comes down to whether one is: (A) annealing every firing for accuracy, or (B) just annealing now and again to keep their brass from dying early..?

If you're in the "B" camp: the mandrel isn't really going to do too much for you, in fact, if you go down to a tight enough bushing to where you then start feeling the mandrel working on non-annealed brass, then you'll definitely be overworking the brass and introducing even more weird randomness with spring-back guesswork.

Annealing every time takes spring-back almost completely out of the equation and IMO is really the only way to really use the bushing/mandrel technique effectively.
If I'm lazy and skip annealing, my mandrel reminds me I'm a horrible person by not doing anything 😜.

FWIW, I use the loaded round OD - .004" for bushing size, then bullet OD - .002" for mandrel size.
 
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