What is the Donut
- Thread starter Poconolg
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big al hunter
New member
The donut sounds like a good way to describe an incipient head separation. I tried to say this the first time and had a technical glitch that I suspect was my phone not cooperating with the server....
Incipient separation usually occurs in bottle neck rifle cartridges. The worst are in rifles that have excessive headspace. The cartridge is pushed forward until the case shoulder is tight to the forward end of the chamber. When the rifle fires the pressure acts on the shoulder and case neck first, locking them to the chamber walls. The pressure then pushes the case back toward the bolt. The case stretches to allow the case head to contact the bolt face. The stretching usually shows as a shiny ring near the case head. This could be described as a donut. You can also feel the groove left on the inside of the case by bending a paperclip into a small L shape and run it up and down the inside of the case. It sometimes causes cracks in the shiny area. And in the most severe examples the case splits completely.
It can also happen in a rifle with headspace that is inside the tolerances. But it takes many reloads and full length resizing without adjustment for the amount of headspace in the rifle. But the exact number of firings depends on how close the rifle is to the tight end of the tolerances, and how far the resizing die is to the loose ends of the tolerances.
Incipient separation usually occurs in bottle neck rifle cartridges. The worst are in rifles that have excessive headspace. The cartridge is pushed forward until the case shoulder is tight to the forward end of the chamber. When the rifle fires the pressure acts on the shoulder and case neck first, locking them to the chamber walls. The pressure then pushes the case back toward the bolt. The case stretches to allow the case head to contact the bolt face. The stretching usually shows as a shiny ring near the case head. This could be described as a donut. You can also feel the groove left on the inside of the case by bending a paperclip into a small L shape and run it up and down the inside of the case. It sometimes causes cracks in the shiny area. And in the most severe examples the case splits completely.
It can also happen in a rifle with headspace that is inside the tolerances. But it takes many reloads and full length resizing without adjustment for the amount of headspace in the rifle. But the exact number of firings depends on how close the rifle is to the tight end of the tolerances, and how far the resizing die is to the loose ends of the tolerances.
Donuts occur on cases that have been neck turned improperly by not cutting into the shoulder slightly. After being fired and sized a small ridge will form inside the neck possibly preventing proper seating. Wilson makes inside neck reamers to remove them, or when neck turning cut into the shoulder very slightly.
That is why cases need to be trimmed before turning the neck. My neck turning lathe indexes off the case mouth. Stop the cut too soon and I get a possible donut, too much of a cut and I have a weak spot at the neck/shoulder junction
edit a 40 sec video the case on the right shows the proper depth of cut to prevent donut formation
https://www.youtube.com/watch?v=N_5vlkgz4ts
I have heard but not experienced that donuts can also form after necking up a case
That is why cases need to be trimmed before turning the neck. My neck turning lathe indexes off the case mouth. Stop the cut too soon and I get a possible donut, too much of a cut and I have a weak spot at the neck/shoulder junction
edit a 40 sec video the case on the right shows the proper depth of cut to prevent donut formation
https://www.youtube.com/watch?v=N_5vlkgz4ts
I have heard but not experienced that donuts can also form after necking up a case
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big al hunter
New member
Learn something new every so often. Just happened. Never turned my necks down, so I never had that one happen. Thanks for the video too.
Donut happens when forming brass from a longer parent, for instance 30-06 to 8mm Mauser. The shoulder of the parent brass got extruded into the new neck. It must be removed by either reaming or turning to be kosher. Unfortunately this step is often omitted.
Having said that, a donut becomes a real risk only when bullet is seated well through one, and the rifle happens to have a tight throat.
-TL
Sent from my SM-N960U using Tapatalk
Having said that, a donut becomes a real risk only when bullet is seated well through one, and the rifle happens to have a tight throat.
-TL
Sent from my SM-N960U using Tapatalk
Don't you mean "tight chamber neck?" Donuts are in the case neck that's in the chamber neck behind the freebore.
The throat starts where the rifling angles down at the front of the freebore.
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Don Fischer
New member
The only donut I've ever heard of in reloading is the one made by sizing down a cartridge to some wild cat. Where I read about it the most was in creating Ackley Imp case's. What happen's it the case is sized down but falls a bit short of the junction of the neck and shoulder. Do that and look at the case and there is a little hump left, donut. As I understand it it is one way of first firing the new case. The donut hold's the case in position for fire forming. The other way I've read about is seating the bullet out into the lands a bit to hold the case in place to fire form.
Don't know if it really works or not, would thing the donut might though. I've sized down a lot of 308 case's to 243 without worrying about putting a donut on them. Also do 30-06 case's and 270 case's to 6.5x06 without a donut, no problem. Might add that in all my rifle's after the first firing, I partical resize for that rifle's chamber. I have 2 243's and bot has their own set of dies set for their own chamber's. One chamber is enough larger that case's for the other rifle won't chamber in it! Might be a good idea to revisit the idea of a donut when sizing down 06 case's to 6.5, maybe!
Don't know if it really works or not, would thing the donut might though. I've sized down a lot of 308 case's to 243 without worrying about putting a donut on them. Also do 30-06 case's and 270 case's to 6.5x06 without a donut, no problem. Might add that in all my rifle's after the first firing, I partical resize for that rifle's chamber. I have 2 243's and bot has their own set of dies set for their own chamber's. One chamber is enough larger that case's for the other rifle won't chamber in it! Might be a good idea to revisit the idea of a donut when sizing down 06 case's to 6.5, maybe!
Donuts can form in any case that has enough reloading and resizing cycles. It happens because the case lengthens when it fills the chamber under pressure and when it is resized to get a shoulder that is closer to the head than as-fired, brass in the shoulder flows into the neck (the thing that causes the need for trimming) and, because it is flowing from a wider diameter to a smaller one, it becomes thicker, especially if the shoulder brass has trouble turning the corner up into the neck as it flows. You can see these on sectioned cases. The inside neck reamers sold for lathe-type trimmers cut donuts out. many people have donuts but don't notice them because they are using a bullet that doesn't occupy the whole length of the neck when they seat it.
The upper case in this picture has a donut forming and you can see reamer marks on the inside of the neck from a previous clearing of the donut. The lower case is once-fired and hasn't been resized and you can see what no donut looks like in that lower image.
The upper case in this picture has a donut forming and you can see reamer marks on the inside of the neck from a previous clearing of the donut. The lower case is once-fired and hasn't been resized and you can see what no donut looks like in that lower image.