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Let's talk about the "dreaded donut"

 
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  #1  
Old 03-09-2011, 09:27 PM
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Let's talk about the "dreaded donut"

The "donut" is a wierd thing. I am frankly a little confused by that theory. If I am understanding correctly, it is a smaller ID right at the neck/shoulder junction.??

Some are saying that it can be created when necking down brass to smaller caliber.

I also heard that the donut is created when necking up brass, which would make more sense (to me anyway). Makes sense that the neck shoulder junction wouldn't want to expand fully or as easily as the neck itself does, and therefore would be a smaller ID once the neck up, or expanding, has been done.

I am failing to visualize how necking down would create a smaller ID at the shoulder junction (based on the same logic of that portion not wanting to move as easily).................Only way this makes sense in my mind is if the necking down was done too small and/or too deep, and then the neck/shoulder portion not fully expanding to the chamber when fired. Seems to me that the case will expand, so long as we're not using a bare minimum or "down loaded" charge?

FWIW, when necking down, I just neck size partially with a neck die (not the full length of the neck) and seat bullets and fire. They chamber just a little snug, because there is now a "false shoulder" where the neck die stopped sizing down, but it's only snug for that first fireing (unless it's a really warm load). I don't recall anyone talking about "donuts" 20 years ago, when I was shooting and forming cases for Benchrest.

I'd like to hear other opinions and logical arguments/reasoning please.
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Old 03-09-2011, 09:54 PM
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Re: Let's talk about the "dreaded donut"

Quote:
The "donut" is a wierd thing. I am frankly a little confused by that theory. If I am understanding correctly, it is a smaller ID right at the neck/shoulder junction.??
The "donut" isn't a theory. It is measurable with pin gauges.

There are many theories about how/why it forms and what can be done to treat or prevent it.

...not that I understand any of those theories beyond speculation

--richard
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Old 03-09-2011, 11:37 PM
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Re: Let's talk about the "dreaded donut"

Quote:
Originally Posted by rscott5028 View Post
The "donut" isn't a theory. It is measurable with pin gauges.

There are many theories about how/why it forms and what can be done to treat or prevent it.

...not that I understand any of those theories beyond speculation

--richard
What are some of those theories if you don't mind?

What are your personal thoughts on the treatment/prevention?
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Old 03-10-2011, 10:11 AM
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Re: Let's talk about the "dreaded donut"

I'll respond just to keep the discussion going because I'd like to learn more. But, I don't have a lot of experience with this issue. So, here's my understanding and hopefully, one of the more knowledgeable will correct me....

The so-called "doughnut" forms on the inside of your case at the intersection of the shoulder and neck. In essence, it's simply a thickening of the brass at that point.

While it may occur during resizing operations to some extent, I think it mostly develops during firing. Due to outward pressure in the chamber, brass flows forward during the firing process and cases stretch and become thinner each time.

As brass flows from the body into the shoulder, the chamber itself (or sizing die) forces the brass to turn the corner inward/down towards the neck. But, when it flows from the shoulder to the neck, there is no solid barrier on the inside of the case to force the brass to turn the corner cleanly towards the neck/chamber.

It mostly turns that corner because of the cohesive properties of a maleable solid. But, some build up/thickening occurs on the inside of the case neck at that intersection with the shoulder.

Many of us are never aware this is happening because it's not enough to prevent the bullet from seating and because our press/expander are not particularly sensitive. But if you use pin gauges, you will find the doughnut after some number of firings.

It's simple enough to ignore and probably isn't going to bother a guy that shoots MOA at 100 yds. But, it affects bullet seating and evenness of your neck tension which are increasingly important as we strive for consistency and precision.

How do we prevent the doughnut? You can't. It occurs regardless of upsizing, downsizing, or using factory cartridges. Having invited argument by saying that, I think the magnitude of the issue is less with necking down and even less again with factory cases/cartridges.

You can exacerbate the problem by necking up which causes necks to thicken. And whether you neck up or not, when you neck turn your cases, leaving a sharp edge where a clean radius previously existed between the shoulder and neck may encourage the doughnut.

As such, a neck turning tool with a perferctly ground radius on the end may help alleviate the issue. But, the real solution is simply to monitor for the presence of the doughnut and to remove/ream it with a fluted cutter pilot tool.

Again, that's my limited opinion. But, I'm eager for someone to help me get it straight.

thanks!
richard
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Old 03-10-2011, 11:40 AM
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Re: Let's talk about the "dreaded donut"

Quote:
Originally Posted by SBruce View Post
The "donut" is a wierd thing. I am frankly a little confused by that theory. If I am understanding correctly, it is a smaller ID right at the neck/shoulder junction.??

Some are saying that it can be created when necking down brass to smaller caliber.

I also heard that the donut is created when necking up brass, which would make more sense (to me anyway). Makes sense that the neck shoulder junction wouldn't want to expand fully or as easily as the neck itself does, and therefore would be a smaller ID once the neck up, or expanding, has been done.

I am failing to visualize how necking down would create a smaller ID at the shoulder junction (based on the same logic of that portion not wanting to move as easily).................Only way this makes sense in my mind is if the necking down was done too small and/or too deep, and then the neck/shoulder portion not fully expanding to the chamber when fired. Seems to me that the case will expand, so long as we're not using a bare minimum or "down loaded" charge?

FWIW, when necking down, I just neck size partially with a neck die (not the full length of the neck) and seat bullets and fire. They chamber just a little snug, because there is now a "false shoulder" where the neck die stopped sizing down, but it's only snug for that first fireing (unless it's a really warm load). I don't recall anyone talking about "donuts" 20 years ago, when I was shooting and forming cases for Benchrest.

I'd like to hear other opinions and logical arguments/reasoning please.
necking up brass properly should never leave a doughnut inside the neck, but I can see where it could happen when reducing the neck size. Some folks will say a 40 degree shoulder will cause it, but you'll see it in a 20 degree shoulder as well. It's brass flow, and brass looking for the path of least resistence. In this case it's the junction of the shoulder and neck. A poorly machined sizing die will also do similar things (or chamber). Some folks think it's caused by 40 degree shoulders, and others don't. I see no connection. But a 30 degree shoulder is easier to form (pressure wise) than a 40 or even a 35 degree shoulder. Yet I see the same doughnut in my generic 22-250 cases (28 degrees), but do think it's more pronounced with a 35 or 40 degree shoulder. It's not the shoulder itself, but where the brass flow stopped. But with my 6/250AI I still see the doughnut, but seems to go a couple firings more it needs to be cleaned out (guess this does prove that a 40 degree shoulder will hinder brass flow).

Lately my case designs are reverting back to the 30 to 35 degree shoulders simply becase they're easier to form, and maybe a tad bit less case shrinkage on fore forming (I have no concrete data to prove this out so far). Also to help create a bit for confusion: I see zero doghnuts with my Weatherby cases!
gary
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Old 03-10-2011, 12:00 PM
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Re: Let's talk about the "dreaded donut"

I have experienced that donut in a drastic application a couple of times. many years ago I wanted an Rimmed 06/30 Gibbs case so purchased some RWS 9.3 X 74 brass and trimmed it to length.

In trimming it to length and then the stepping down to 30 caliber the brass in the body/shoulder area of all cases is much thicker. So mechanically and prior to firing one has formed a smaller dimension (donut) just because that is what happens with that thick part of the case. With a high energy cartridge like the Gibbs one uses load data published for the 30 Gibbs when one may very well have a 284 diameter inside that case. As one shoots and the case forms to the chamber more material swells in that area further decreasing that area.

Yes I saw my pressures spike after a brief period of time and when checking I pinned a 6.5 diameter (roughly) and I was loading for a 30 caliber. So yes that donut is a pain but it is also very dangerous if one doen't know it is there. I have cutters and just ream the donut out. It will come back so I switched cases to 7X65R and never saw another donut again.

I am currently working with the 5.6 X 57 RWS case and the neck of that case is extremely thick. I am waiting for tooling so I can turn down the neck/shoulder area in hopes of avoiding the pain in the butt donut. There is such a drastic thickness I will need to turn down the outside and after forming the case I will ream the inside.

The reason I would go through something like this is because the RWS case are the toughest cases I have ever used. Quality is as good as it gets and they use to cost a buck a case. Years ago I purchased a life time supply of different RWS cases and haven't lost a case yet. They are as tough as a WSM case without being brittle.

For those interested, Huntingtons is the only company I know of currently importing RWS case's.

Neal

Last edited by hammertyme; 03-10-2011 at 12:08 PM.
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  #7  
Old 03-10-2011, 12:27 PM
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Re: Let's talk about the "dreaded donut"

SBruce, it happens with 2 events:
1. Up-sizing necks
2. Alot of FL sizing, especially on poorly designed cases(like a 30-06)

#1 Brass tapers in thickness -thickest near the webs to thinnest at the mouths. With up-sizing you're now turning a bit of thicker shoulder brass into thinner neck area. The same happens with case forming longer necks.

#2 FL sizing begins lowest on the case. As the case is further sized it is higher and higher up the case to & through the neck. It's like squeezing toothpaste because down sizing anywhere thickens brass -in a thinner brass direction.
This is why you have to trim all the time with a 30-06. That toothpaste tube lengthens as the brass slowly works it's way up the case, lengthening them.
Those with low taper/high shoulder angle cases(AI) trim less because their brass isn't squished so upwardly but more inwardly(has nowhere else to go).
Those who size a minimal amount either with custom dies and/or with neck sizing only will also reduce the need for trimming. They aren't squishing brass any direction at all.

Brass is put where it is by SIZING,, not firing.
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