A doughnut is a thickening at the base of the neck, where it meets the shoulder that is caused by anything that causes the thicker shoulder brass to move up into the base of the neck. When chambers that are designed for a close fit to loaded rounds (with either turned or unturned necks) are combined with bullets that are seated so that their shanks (the full diameter part of the bullet) are in the same area that this thickening has occurred, the diameter of the loaded round may be larger than the chamber (within a narrow band), causing excessive pressure, that can degrade accuracy, and which also may be a safety issue. If the chamber has more room, there may still be a problem with increased bullet pull that varies from round to round due to non-uniformity of the problem.
Now we get to the root of the problem. How do doughnuts come about? There can be more than one cause. Let me give you an example. When I make Lapua .220 Russian cases into 6PPCs, in order not to have excessive bolt closing force when fire forming, I have to turn a little bit (note precise terminology) into the case shoulder. If I don't do this, and simply force the bolt to close, the case will have a doughnut, without ever having been fired or sized, because the chamber, and bolt, acting like a one piece die,and shell holder, forced some of what started out as the shoulder of the case, up into the neck. Of course this all takes place without the case ever seeing a bushing.
In normal use, when a case that did not have a doughnut after it was fire formed, is sized in a FL die that reduces its body diameter, and is set to bump back its shoulder, the brass from the body diameter reduction, and shoulder bump has to go somewhere. It gets shoved into the base of the neck when the case is sized.
Cartridge design also gets into the mix. Cases that have smaller shoulder angles can be more easily forced forward in their chambers under the force of a heavy firing pin fall, and the force that the primer's explosion exerts on the bottom of the primer pocket. When this happens, the subsequent force of the igniting powder can cause the case to stick to the chamber for most of its length, and if the pressure peak is sufficient, the case will stretch, just in front of it's head, as the head comes back to the bolt face. With cases of this design, I have found it a good idea not to bump the shoulder any from its first or second fired condition (I set the die so that it measures the same.), when full length sizing, in order not to have a thinning problem (known as an incipient separation) where the case has been forced to stretch when fired. In an informal way, I have said that this may reduce the amount that the case is knocked forward by the firing pin and primer, by not letting it "get a run at it" . It is hard to bend something with a hammer if you are not allowed a back swing.
In any case, I have said this by way of explanation of why I disagree (respectfully, and I hope politely) with the idea that bushing dies cause doughnuts. The section of the neck that the bushing does not reach in just that. It may or may not be coincident with a doughnut, and IMO, one does not cause the other.
Some time ago, I ran across a phrase. Correlation does not prove causation. On a lighter and less respectful note, I have sometimes made reference to the "cargo cult" of some the New Guinea aborigines.
Boyd
Boyd
Yes, they all full length size ;D.
