Necking down brass question

[Gregor.Samsa]

When necking down brass, I'm assuming you would anneal prior to the actual re-sizing process. Would you want to anneal again after sizing to relieve the stress that was just introduced? I'm also reading conflicting information regarding the need to neck turn. I'd really like to avoid going down this path if at all possible. I'm not planning on doing anything extreme, just transforming 6.5cm to 6cm. thanks

 

[Threadcutter308]

I shoot 6x47 Lapua, which means I have to neck down from 6.5 to 6mm. I honestly couldn't tell you if I have annealed first, it's such a small change and I have to fireform anyway, I don't worry about annealing before reduction. Also, I neck turn for all my bolt guns.
Seems like a lot of people get their panties in a wad over neck turning. What everyone seems to lose sight of is that neck turning only has to be done once. Now, it you are shooting comps and losing some percentage of brass each match, then yes, neck turning can be a pain where you don't need it.

 

[Steel head]

Usually necking down doesn’t require neck turning because thin neck material is becoming shoulder.
If the brass is new just go for it otherwise I’d anneal before.
I would definitely use imperial lube, it’s the best.

 

[Gregor.Samsa]

Thanks all. I'd like to avoid neck turning as I don't have any of the equipment and for my level of shooting I think its diminiushing returns

 

[918v]

What everyone seems to lose sight of is that neck turning only has to be done once.

Maybe with your cartridge. But that is certainly not the case with most.

 

[Threadcutter308]

.223, 6x47, .308W, all bolt guns. I've only ever turned brass once. Specific brass lives with a specific rifle. There may be other situations (gas guns ?) where multiple instances of turning on the same brass is necessary, I've just never seen them.

 

[phlegethon]

People will say a lot of things about this, that you need to anneal, that you need to use a nonbushing die, whatever. I’m sure that’s true in some situations. I can tell you I ran 500 pieces of 6.5 Creedmoor brass through a Hornady bushing die to 6 CM, with just Hornady One Shot lube, not annealing, and it worked fine for PRS.

 

[ZiaHunter]

In necking down 6.5CM Lapua to 6CM measured before and after neck wall thickness and there was an increase. Necked down with FL standard die and did end up turning to allow sufficient clearance in the chamber. Like Threadcutter I turn all my brass for my F Class and ELR rifles in which the brass is specific to one rifle. Only turn during initial case prep and using a 21st Century neck turning tools goes pretty quick.

 

[Gregor.Samsa]

I plan on running a mandrel through after necking down. Should I account for more spring back than usual and use an expander vs turning mandrel?

 

[phlegethon]

I plan on running a mandrel through after necking down. Should I account for more spring back than usual and use an expander vs turning mandrel?

Why not just try both and see what the seating feels like?

 

[Ledzep]

The short answer:
6.5cm to 6cm I would run it through a FL size die with an expander ball and don't think twice about it. Easy peasy.


The long answer:
As you neck down, neck wall thickness theoretically will increase. As you neck up neck wall thickness will decrease. Assuming the case doesn't grow or shrink, you can use the cross-sectional area of the "before" to get the OD radius of the "after".

C/A = pi*(Ro^2 -*Ri^2)
Ro is the OD of the neck divided by 2, Ri is the ID of the neck divided by 2.

C/A before = C/A after

ALGEBRA TIME!
Pi* (Rob^2-Rib^2) = Pi* (Roa^2-Ria^2)
Drop Pi from both sides

Rob^2-Rib^2 = Roa^2-Ria^2

We know the ID of what we're going to (mandrel/expander/bullet diameter) so to find the Roa (radius of the outside "after") just add it to both sides and square root the whole thing.

Roa= SQRT( Rob^2-Rib^2+Ria^2 )

OD after = 2* Roa

Rob = .146 <---I made that number up as an example
Rib= .132 (this is .264/2)
Ria= .1215 (this is .243/2)

then Roa= .1366

So where you started with .014" neck wall thickness (Rob-Rib), you would theoretically end up with .0151" neck wall thickness after stepping down to 6mm (Roa-Ria)... .001 difference wooo big deal.

That all assumes that your cases don't grow in length, too. If they grow length-wise, then whatever volume of material flows up the neck will not contribute to increased neck wall thickness. Depends on lube, die design, etc.

At any rate, with the diameter reduction of only ~8%, you probably don't need to worry about annealing until after several firings.

 

[Gregor.Samsa]

Why not just try both and see what the seating feels like?

yeah have both coming in the mail.

 

[Gregor.Samsa]

The short answer:
6.5cm to 6cm I would run it through a FL size die with an expander ball and don't think twice about it. Easy peasy.


The long answer:
As you neck down, neck wall thickness theoretically will increase. As you neck up neck wall thickness will decrease. Assuming the case doesn't grow or shrink, you can use the cross-sectional area of the "before" to get the OD radius of the "after".

C/A = pi*(Ro^2 -*Ri^2)
Ro is the OD of the neck divided by 2, Ri is the ID of the neck divided by 2.

C/A before = C/A after

ALGEBRA TIME!
Pi* (Rob^2-Rib^2) = Pi* (Roa^2-Ria^2)
Drop Pi from both sides

Rob^2-Rib^2 = Roa^2-Ria^2

We know the ID of what we're going to (mandrel/expander/bullet diameter) so to find the Roa (radius of the outside "after") just add it to both sides and square root the whole thing.

Roa= SQRT( Rob^2-Rib^2+Ria^2 )

OD after = 2* Roa

Rob = .146 <---I made that number up as an example
Rib= .132 (this is .264/2)
Ria= .1215 (this is .243/2)

then Roa= .1366

So where you started with .014" neck wall thickness (Rob-Rib), you would theoretically end up with .0151" neck wall thickness after stepping down to 6mm (Roa-Ria)... .001 difference wooo big deal.

That all assumes that your cases don't grow in length, too. If they grow length-wise, then whatever volume of material flows up the neck will not contribute to increased neck wall thickness. Depends on lube, die design, etc.

At any rate, with the diameter reduction of only ~8%, you probably don't need to worry about annealing until after several firings.

Makes sense. Thanks for this. Assuming this is a saami chamber, the increase in neck thickness shouldn't affect chambering correct?

 

[Ledzep]

Usually not. If you take Lapua or military surplus .308 brass and make .260 or .243 out of it sometimes you can have problems but typically the SAAMI chambers allow for quite a bit of neck thickness to cover manufacturing tolerance/practice.