Let's talk about the "dreaded donut"

Discussion in 'Reloading' started by SBruce, Mar 9, 2011.

  1. SBruce

    SBruce Well-Known Member

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    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.
     
  2. rscott5028

    rscott5028 Well-Known Member

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    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
     

  3. SBruce

    SBruce Well-Known Member

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

    What are your personal thoughts on the treatment/prevention?
     
  4. rscott5028

    rscott5028 Well-Known Member

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    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
     
  5. Trickymissfit

    Trickymissfit Well-Known Member

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    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
     
  6. hammertyme

    hammertyme Well-Known Member

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    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: Mar 10, 2011
  7. Mikecr

    Mikecr Well-Known Member

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    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.
     
  8. rscott5028

    rscott5028 Well-Known Member

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    Mikecr,

    If I understand you correctly, then I respectfully disagree with that statement.

    Measure your case. Fire it. Measure again. It will expand to the chamber, springback slightly, and lengthen. ...flowing from thick to thin, bananas, doughnuts, and all compounded by resizing and further work hardening

    I apologize if I'm taking that too literally or out of context.

    -- richard
     
  9. woods

    woods Well-Known Member

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    I have to disagree also. I check all my cases with pin gauges and have only run across 1 case in which I develop a do-nut; with Nosler 280AI brass. The do-nut is detectable on once fired cases

    [​IMG]
    [​IMG]

    These case have not seen a resizing die yet. The new Nosler 280AI cases have necks that are not dented up and need no sizing prior to loading.

    So do-nuts can develop from firing only.
     
  10. SBruce

    SBruce Well-Known Member

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    Thank you all for the responses.

    After reading them, I can visualize how donuts may occasionally form from firing and case stretch/brass flow. I experienced alot of brass flow and stretch with a 220 Swift, but I never pin guaged the neck ID to know if I had donuts forming or not.? I didn't however notice any loss of precision or opening of groups until the barrel was getting pretty long in the tooth. But I neck sized (and only partial) most of the time, only FL sizing and trimming about every 3rd fireing. Perhaps that helped, guess I'll never know for sure. The Swift has alot of body taper in addition to a lesser shoulder angle. More prone to stretching/flowing even over the 30-06 IMO.

    Mikecr, your comments are making alot of sense too, and are along the lines of what I was originally thinking. Kinda confirming that donuts are also occasionally created by the sizing process used. I have certainly noticed longer cases when FL sizing, even longer still when FL sizing with an expander ball.

    My latest wildcat is a 6 Long Dasher, formed by necking down from 6.5X47 Lapua, then further formed into a longer body with a 40 degree shoulder once fired. So far, my knowledge of this wildcat is limited because I've only loaded and fired one case 12 times, and another case 3 times. I took multiple measurements (OD's and lengths in multiple spots) after every fireing/charge increase. I increased the charge 1/2 grn at a time until I found absolute max charge and consequently ruined the case. I saw absolutely no lengthening of the case during the process.............kinda confirming that sharper shoulder angles stretch less upon being fired.

    If I am understanding you all correctly, the only way to remove the donut if one should arrive, is to ream the ID of the neck?

    I used to read and hear that inside neck reaming creates it's own problems/issues so I've never done it. Any comments on that?
     
  11. rscott5028

    rscott5028 Well-Known Member

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    I suppose there are extreme cases where this could be a big accuracy problem or even dangerous. But, it's mostly benchresters that worry about this and their matches are won or lost by hundreths of an inch.

    I suppose that might depend on whether you are simply reaming the doughnut or trying to ream inside the neck for thickness.
     
  12. SBruce

    SBruce Well-Known Member

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    I believe they were relating to reaming for thickness. Reasoning is that I'd never heard of the "donut" untill I began reading these forums just a little over a year ago.

    I am handling the thickness by turning the outside necks for this particular rifle (it is a tight neck chamber).
     
    Last edited: Mar 10, 2011
  13. Mikecr

    Mikecr Well-Known Member

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    Woods
    I can't think of an action that would cause expanding brass(thinning) to SQUISH eleswhere.
    Are the new(unfired) neck ODs tapered from mouth to shoulder?
    Then are your formed neck ODs tapered?

    I ask because I know tapered necks are somewhat common with hunting chambers, and new brass. A taper could allow thicker brass near the shoulders than nearest the mouths, without stopping a pin gauge.
    With this, any outside taper left inward with fire forming might then stop a pin gauge.
    If this is the case and it's an actual problem, I would neck turn before fireforming to control it.
    But otherwise, it's still a mystery to me.

    Richard, when brass lengthens soley on firing, it's not 'flowing'. It's 'stretching' backward toward the boltface, thinning all the way, and can eventually lead to seperations.
    This is just opposite -in every respect, to what is going on with FL sizing.
    In fact, if anything, this lengthening/thinning would in itself act to reduce an existing doughnut.
    It's when you squish it all back with sizing, from base toward the neck, that brass is pushed that direction.
     
  14. Trickymissfit

    Trickymissfit Well-Known Member

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    Steve,
    once you get used to using gauge pins you'll never go back! You can get them in different grades, and in sets of different sizes. At one time I had them all the way out to an inch, plus all the letter and number drill sizes (you can also order metric sets too). If I remember right they are accurate to within .000050". Another way to do the same thing is with small hole gauges that expand, and are ment to be measured with a micrometer. They make two different styles, and one is ball shaped and the other is d shaped. I like the latter better because you can do more with it.
    gary