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Please Explain Donuts to me

Dimner

I do believe in Captain Crunch.
Hi guys. I've tried searching for my answers but half the threads turn into an argument with fguffy, the other half devolve into how to fix a donut or other topics. It's hard for my head to collate what I am reading into a cohesive set of information that I can use to for a decision.

Those old threads are just fine on how to fix a donut...but the basics are lacking.

So with that...

What is a donut? (I actually think I have this part down)

How does it form?

How do I identify if I have one?

I'm using standard cartridges that are multi fired from virgin brass. Nothing has been formed from another case. The cartridges I'm concerned with are 30-06, 308, 7x57, and 223. I thought donuts were only an issue with necked up brass, but now I'm finding conflicting information.
 
Dimner said:
Hi guys. I've tried searching for my answers but half the threads turn into an argument with fguffy, the other half devolve into how to fix a donut or other topics. It's hard for my head to collate what I am reading into a cohesive set of information that I can use to for a decision.

Those old threads are just fine on how to fix a donut...but the basics are lacking.

So with that...
Click to expand...
As someone who use to work forming metal for aircraft part, let me give you my understanding of what this is and how it's formed.


Dimner said:
What is a donut? (I actually think I have this part down)
Click to expand...
It's the very slight thickening of the metal at the neck-shoulder junction where it's a narrow band that protrudes into the interior of the neck. It narrow the opening into the shoulder area.

Dimner said:
How does it form?
Click to expand...
Often, it's portrayed as the result of brass flow from firing. From the testing and measuring that I've done, I find that it really do to the sizing process and the firing process has very little direct contribution.

When you size the case, the brass doesn't just flow in one direction, particularly with the neck where the material typically being moved the most (mostly in the direction of least resistance). So, when the neck is being sized from the mouth to the junction at the shoulder as the case moves into the die, neck material flow mostly up towards the mouth and some is forced down as the die moves over the neck. This flow downward stops at the neck-shoulder junction due to the change in the angle where that flow encounters an increasing diameter and mass (the shoulder acting like a dam) creating an area at that junction that's thicker and protrudes only to the interior since the sizing die prevents any of this flow to the outside. If one is using a bushing die, not only will there be an area on the neck at that junction that isn't sized, but the exterior in that area will also show a little growth due to that flow.

Dimner said:
How do I identify if I have one?
Click to expand...
It can be felt when seating a bullet, depending on the nature and type of press one is using. Like the bullet will start move with uniform pressure as the bearing surface moves through the neck, then you can feel the increased resistance when the bullet then touches it.

If it's pronounced enough, shining a bright light into the neck and it can be visible.

Dimner said:
I'm using standard cartridges that are multi fired from virgin brass. Nothing has been formed from another case. The cartridges I'm concerned with are 30-06, 308, 7x57, and 223. I thought donuts were only an issue with necked up brass, but now I'm finding conflicting information.
Click to expand...
Using an expander mandrel, as opposed to an expander ball in the sizing die, does a good job of mitigating any problem with a donut, pushing it outward. Better yet, just not seating bullets to where they contact the donut with the bearing surface works best. ;)
 
Donuts don't really matter until the bullet is pushed in the case deep enough to reach the junction between the neck and the shoulder. That is the reason most guns are throated so the bullet doesn't reach the donut.

They can be there on Virgin brass.

I use a pin gages tgo identify their presence.
 
Here’s my thinking. Sized brass is fired and the body, shoulder and neck of the bottleneck case expands to the size of the chamber. Once one begins to resize it and it begins being forced into the die the first contact is at the body/shoulder junction and possibly the neck as the case moves into the die. The brass has stretched and thinned slightly during the firing and the overall length DIDN’T reduce. The brass always flows from the case head towards the muzzle in the chamber and the same while being sized. The neck and body are being reduced and the brass MUST flow towards the neck and up out of the die. As the shoulder contacts the inside of the die, it’s pushed in towards the center and must also flow vertically but because there’s no mandrel inside the neck it thickens at the shoulder neck junction. Inserting an expanding mandrel after the sizing die forces some of this thickened area outward and possibly downward but spring back occurs after the mandrel is removed and there’s still a thick smaller diameter area present at the shoulder neck junction that can only be removed with a ream. I always have at least enough free bore available to allow the bullet bearing area to be seated only in the neck above the donut. I modify the barrel with a throat ream if it won’t allow this bullet seating depth.
 
Straightshooter1 said:
As someone who use to work forming metal for aircraft part, let me give you my understanding of what this is and how it's formed.



It's the very slight thickening of the metal at the neck-shoulder junction where it's a narrow band that protrudes into the interior of the neck. It narrow the opening into the shoulder area.


Often, it's portrayed as the result of brass flow from firing. From the testing and measuring that I've done, I find that it really do to the sizing process and the firing process has very little direct contribution.

When you size the case, the brass doesn't just flow in one direction, particularly with the neck where the material typically being moved the most (mostly in the direction of least resistance). So, when the neck is being sized from the mouth to the junction at the shoulder as the case moves into the die, neck material flow mostly up towards the mouth and some is forced down as the die moves over the neck. This flow downward stops at the neck-shoulder junction due to the change in the angle where that flow encounters an increasing diameter and mass (the shoulder acting like a dam) creating an area at that junction that's thicker and protrudes only to the interior since the sizing die prevents any of this flow to the outside. If one is using a bushing die, not only will there be an area on the neck at that junction that isn't sized, but the exterior in that area will also show a little growth due to that flow.


It can be felt when seating a bullet, depending on the nature and type of press one is using. Like the bullet will start move with uniform pressure as the bearing surface moves through the neck, then you can feel the increased resistance when the bullet then touches it.

If it's pronounced enough, shining a bright light into the neck and it can be visible.


Using an expander mandrel, as opposed to an expander ball in the sizing die, does a good job of mitigating any problem with a donut, pushing it outward. Better yet, just not seating bullets to where they contact the donut with the bearing surface works best. ;)
Click to expand...
Thank you very much for that detailed answer. I don't use bushing files, I use the forster BR dies with the high expander ball. Will this help or hurt or affect anything?
 
mgunderson said:
Donuts don't really matter until the bullet is pushed in the case deep enough to reach the junction between the neck and the shoulder. That is the reason most guns are throated so the bullet doesn't reach the donut.

They can be there on Virgin brass.

I use a pin gages tgo identify their presence.
Click to expand...
Unfortunately I have a near zero throat 30-06 I'm dealing with and so my hornady 178 ELD-x bullets have to be seated crazy deep. So I'll try and see if I can feel any extra pressure needed when I get to the neck/shoulder junction
 
Dimner said:
Thank you very much for that detailed answer. I don't use bushing files, I use the forster BR dies with the high expander ball. Will this help or hurt or affect anything?
Click to expand...
I think that your Forster dies would help. If a donut even tried to form with them, the expander ball should pull it out of the neck to be trimmed off as extra length.
 
I have had doughnuts before on my brass, the last time I resized some cases I tried two different resizing methods and was surprised by the results.

Starting with 50 pieces of Starline 223 brass fired 6 times in my 223 bolt action Remington 700.

Method 1: Resized 25 pieces of 223 brass using a non bushing RCBS FL die that has the neck honed, after resizing I pushed a turning mandrel into the neck.

Checking the neck with a .221 pin gauge the gauge would stop at the neck shoulder junction confirming a doughnut had formed.

Method 2: Resizing 25 pieces of 223 brass using the same non bushing RCBS FL die that has the neck honed but with a Forster expander assembly installed in the RCBS die (expander button is high position in the die).

Checking the neck with a .221 pin gauge the gauge would slide completely through the neck confirming that NO doughnut had formed.

So Mandrel = Doughnut and Expander button = No Doughnut.
 
Kracken McGraw said:
I think that your Forster dies would help. If a donut even tried to form with them, the expander ball should pull it out of the neck to be trimmed off as extra length.
Click to expand...
The expander balls I have used has never pulled a doughnut from the neck shoulder junction. Once it forms it’s there
 
Looks like it's time to invest in some pin gauges! Do you guys know if there is anyone that sells a cheap 30cal set and a 7mm set? Or a set from something like. 250 to .325?
 
Dimner said:
Looks like it's time to invest in some pin gauges! Do you guys know if there is anyone that sells a cheap 30cal set and a 7mm set? Or a set from something like. 250 to .325?
Click to expand...
You don’t need them to tell if you have doughnuts or not
 
DLT said:
If a bullet will not drop in a fired case you probably have doughnuts
Click to expand...
I’m not sure that’s 100% accurate
That could be a neck thickness to chamber neck issue.
If bullets drop almost all the way down the neck but get stopped or resistance at the neck/shoulder junction that would be a sign of donuts.

You really feel it when seating Bullets.
As the shank of the bullets gets to the donut a distinct increase in resistance is felt.
If your shank stops forward of the donut the donut is irrelevant
 
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