BRASS SPRINGBACK

[mikecr]

What you're seeing is the reason it's better to turn necks with new/unformed cases, just neck expanded only.
Do this even if the new case cal is larger than intended. That is, if your intending to downsize 6.5x47 to 6x47, turn the necks at 6.5cal, which will take a few culled in trial & error.

There is no need for pin gauges to accurately measure necks, just as there is no need for reamers to turn necks. Just a properly setup neck mic, calipers, and a good neck turning system.

 

[Brians356]

There is no need for pin gauges to accurately measure necks, just as there is no need for reamers to turn necks. Just a properly setup neck mic, calipers, and a good neck turning system.
View attachment 978659

Uh, who said I use pin gages to measure neck walls? I have a .0001 Mitutuyo tubing micrometer for that. But pin gages allows you to judge which portions of the irregularities are on the inside vs outside of the neck.

Did I say a reamer was necessary to turn necks? I have a lathe-style neck turning kit for that, which can produce neck walls with <= .0003" variation.

What I was pointing out was that necks develop odd thickness variations after sizing down significantly. I'm seeing that these are difficult to "iron out" using expander mandrels, and therefore may not be addressed satisfactorily by outside neck turning alone (which assumes irregularities have been forced to the outside of the neck by the expander.) In particular, reamer dies were designed to attack extreme examples of this, in particular to remove donuts resulting from shoulder material being forced into necks when forming shorter cases from longer ones.

Your suggestion to neck turn before downsizing necks is reasonable, and thanks. However, it does make it a trial-and-error process to discover what the final neck wall thickness will be after downsizing.

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[ireload2]

Hello Brian
I have several of the RCBS reamers and reamer dies that I got with forming die sets.
I have a set that forms .250 Savage from .30/06 or similar brass. The reamer die neck is .284 (verified with gage pin set). The reamer is .2575 diameter. The reamer shank is the same diameter and it is guided by a close fitting diameter in the top of the reamer die body.

Probably the best way to turn brass is to turn a mandrel in a lathe and leave it. Push the cases on with the tail stock and turn.
When done in this manner there is zero clearance between the mandrel and and the inside of the case neck. Even if the neck has internal stress it is forced into what is called "perfect form" for turning.

I see that even before expanding. Last night I formed a case from 7x57 -> 257 Rob -> 6mm Rem, and carefully measured the neck. Right at the neck/shoulder junction there is a slight bottleneck, the neck OD is ~.001" smaller than .050" closer to the case mouth, where there is a slight donut that swells the outside as well as the inside. From there the neck walls get gradually thinner toward the mouth, the difference in thickness between the donut and the mouth is ~.005"!

This suggests the best way to deal with necks and donuts when forming down from larger necks is to employ a proper ream die and reamer. I've never seen that setup, but I would assume the ream die squeezes the neck OD into compliance, and the reamer is controlled and properly centered to produce reasonably uniform neck walls. (I may have an overly optimistic impression of how it works.)

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[Brians356]

Thanks for the reamer die details.

Probably the best way to turn brass is to turn a mandrel in a lathe and leave it. Push the cases on with the tail stock and turn.
When done in this manner there is zero clearance between the mandrel and and the inside of the case neck. Even if the neck has internal stress it is forced into what is called "perfect form" for turning.

In that setup the case is wedged onto, and turns with, the mandrel?

 

[Brians356]

What you're seeing is the reason it's better to turn necks with new/unformed cases, just neck expanded only.

I notice some chaps say they fireform new cases at least once before neck turning. What's the reasoning behind that (anybody)?

I like your idea, though, as it seems to me the case will never again be so devoid of any trace of a donut.
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[ireload2]

yes, the case is actually driven by the friction of the mandrel.

Thanks for the reamer die details.



In that setup the case is wedged onto, and turns with, the mandrel?

 

[Webster]

Thanks for the reamer die details.



In that setup the case is wedged onto, and turns with, the mandrel?

I would think that if you force a mandrel into a case neck you would get galling or some texture change? I never really looked for surface texture change when forcing a K&M expander into necks. It's been a long time since I had new cases. If I remove donuts I'm doing it with carbon in the necks. What's your feeling on forcing mandrels into a neck and changing the surface?

 

[mikecr]

I notice some chaps say they fireform new cases at least once before neck turning. What's the reasoning behind that (anybody)?
I like your idea, though, as it seems to me the case will never again be so devoid of any trace of a donut.
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Neck turning is very difficult to screw up, but there are always those expending efforts to do so..
The only notion I can come up with for fire forming first, is to set shoulders to specific angle (like from 243win to 243AI & maybe ~matching their cutter). But this isn't critical at all, and their formed shoulders won't actually match the cutter anyway. They're better to simply pick up a cutter with angle nearly matching new unformed cases, and run the cutter to kiss the neck-shoulder junction with a bit of common sense.
Once they've fired a neck -first- it would have to be full length resized back to fitting their turning mandrel, adding energy & springback to counter this extra effort here. Firing will push thickness inward, downsizing will again push it inward, while the function of turning relies on thickness being outward. Your turning expander can only work so well to undo all this energy(beyond design intent).
So some of these folks who insist that cases be fireformed first -end up insisting that reamers are needed.
It's classic tail chasing.

All new cases have donuts, it's inherent to case manufacture. A 'problem donut' is formed as the neck grows on firing, putting thicker neck-shoulder brass into the formed neck. One of the purposes of neck turning is to mitigate this through turning relatively thicker neck-shoulder brass away. You take a new neck, run the expander mandrel through it, and most of the neck-shoulder thickness is exposed to the cutter. We're talking 1-2 thou, and it takes only a slight touch of the cutter to remove it. Those who cut way down onto shoulders fall into the tail chaser category.
I don't even use a 'stop' with my turner, I don't trim before turning, I just watch what I'm doing. I also measure my results the best I know how and they're fine. So it just seems difficult(to me) to make trouble of it.

Inherent donuts are not a problem until you seat bullet bearing into them or FL size necks. If your load needs extra bullet grip, then these will damn sure provide it. Otherwise, your load will probably present greater ES with this. More grip means more variance in it.