I'm trying to get my head around the issue of the dreaded donut. I'm planning to turn necks (up to and a little into the shoulder) and then neck-size cases with a bushing die (L.E. Wilson neck bushing die) that resizes only the last 3/16" of the neck, leaving the portion near the shoulder unsized. Under these conditions, will I encounter the donut problem at some point? Can someone explain this to me?
Understanding the "Dreaded Donut"
- Thread starter South Pender
- Start date
This is basically my method of turning and sizing but it is best to fire-form the brass first IMO. I have seen no donut issues to date.
Lots of talk about donuts but they aren't a big deal as long as the bullet isn't seated into or below that portion of the neck. If you use a bushing die and don't size that portion of the neck, most of the donut stays on the outside, depending on neck clearance. This can help align the round in the chamber.
As i understand it, brass gets warm enough to start to flow when the cartridge is fired. edit: it sounds to be from movement of brass when resizing after the brass stretched to chamber dimensions when fired. Depending on the the design of the case, it gets deposited in the shoulder of the case. Some case designs get fewer donuts. If enough brass flows forward, some of it deposits at the neck/shoulder junction and forms a ring.
If you cut just a tiny bit into the shoulder, that leaves an area for the brass to flow in to. If that part of the neck doesn't get resized, it shouldn't push the ring back in to the inner diameter of the neck where it would interfere with bullet seating. Inside neck reamers exist but I don't use them and most people probably don't.
Maybe someone will have some pictures of cases that have been cross sectioned to visually show the accumulated brass. I would think any case, at least in theory would get donuts if fired enough times. Longer necks, a longer throat and treating the cases as you describe should hopefully reduce problems. I would say, especially if you consider them when planning your setup, you shouldn't be concerned.
If you cut just a tiny bit into the shoulder, that leaves an area for the brass to flow in to. If that part of the neck doesn't get resized, it shouldn't push the ring back in to the inner diameter of the neck where it would interfere with bullet seating. Inside neck reamers exist but I don't use them and most people probably don't.
Maybe someone will have some pictures of cases that have been cross sectioned to visually show the accumulated brass. I would think any case, at least in theory would get donuts if fired enough times. Longer necks, a longer throat and treating the cases as you describe should hopefully reduce problems. I would say, especially if you consider them when planning your setup, you shouldn't be concerned.
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I agree with most of that and will add that anytime a case is necked up, part of the thicker shoulder brass becomes part of the larger neck.
I made some 6xc cases out of 308 cases once. The body brass is thicker than the shoulder brass, which is thicker than neck brass. I got big donuts after the first firing because there was so much brass that moved into the neck area.
There is very little, if any, movement of brass during firing. Virtually all of the brass movement occurs during sizing. I always cut the throat to keep the bullet ahead of the neck/shoulder juncture so as to avoid any doughnut issues. WH
Neck bushings do not contribute to donut area thickness.
But ALL brass comes with slight donuts, as it's inherent to manufacture.
New brass tapers in thickness from webs all the way to mouths.
When you turn NEW cases you'll remove the taper & mitigate donut issue -until you've sized brass back into necks (from below). If you have to trim at some rate in reloading cycles, then you're moving brass thick toward thin right up the case and back into necks. Best to keep bullet bearing clear of this if it get's to the point of issue, and NEVER size it (bringing it into tension).
To mitigate:
But ALL brass comes with slight donuts, as it's inherent to manufacture.
New brass tapers in thickness from webs all the way to mouths.
When you turn NEW cases you'll remove the taper & mitigate donut issue -until you've sized brass back into necks (from below). If you have to trim at some rate in reloading cycles, then you're moving brass thick toward thin right up the case and back into necks. Best to keep bullet bearing clear of this if it get's to the point of issue, and NEVER size it (bringing it into tension).
To mitigate:
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Seems from your explanation that doing neck-only sizing with a bushing neck die would prevent brass flowing from the shoulder into the neck.
BoydAllen
Gold $$ Contributor
There seems to be a widely repeated and incorrect idea of where doughnuts come from. When a case if fired for the first time there is more clearance than is the usual for a properly resized case (for a bolt action). The firing pin strike knocks the round forward in the chamber until the shoulder of the case is stopped by the shoulder in the chamber. From that point the ignition of the powder and attendant pressure causes the case to expand, creating sufficient friction between the body of the case and that part of the chamber that any stretching of the case back to the bolt face happens toward the back of the case. (Look up incipient separation if you want to know more about this.) This causes a slight thinning at the back of the case in a narrow zone, but if you are careful setting your die for shoulder bump, it should not be a problem....except that for every cycle of firing and FL or body sizing brass from the body is moved forward in the case. This is caused both by the bump and the reduction of the diameter of the body of the case. This is what causes cases to grow in length, and require trimming. As a part of this process the thicker brass at the top of the shoulder is moved into the base of the neck and when that happens we have doughnuts. It is not a matter of brass flowing during firing. Some may have noticed that their unturned necks are thicker at their base than at their mouths. This is normal. If you section a case lengthwise you will see that it has a continuous taper in wall thickness from the head to the mouth. That is why most factory chamber drawings show taper in the neck, so that if the bearing surface of a loaded bullet is seated deep that chamber clearance will not decrease toward the bottom of the neck. On the whole neck sizing only thing, if you are loading to normal pressures, eventually cases will become excessively tight. When they do, it will not happen on the same firing for all cases and if you shoot tight with not tight, your groups will be enlarged over what they would be if they were all tight or all not. You can also end up with lug galling issues from running tight brass, especially if your lugs are not lightly greased on their rear surfaces on a regular basis.
Great explanation, Boyd. Many thanks. I get it that neck-only sizing will in time lead to overly-tight cases on chambering. My question was more whether, with neck-only sizing, one would avoid the donut (at least before FL or body sizing to restore light chambering).
Bill in some chambers you have turn the brass in order to chamber a round for fireform.
Absolutely. Sorry. Yes I was referring to no-turn chambers only.
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