Yikes. Gremlins. Case neck “donuts” are a common development in an aging cartridge case, and it’s often unknown. Read this and know! MORE
Even if the case neck passes the “drop test,” there might be something amiss within that cylinder, and it might not show up until after case sizing, and that is the “dreaded donut.”
What exactly is a case neck donut? It’s a tiny elevated ring of brass on the interior circumference of the case neck, right at the juncture of the case neck, case shoulder. It is pretty much a little o-ring, in effect.
This “tight spot” reduces the case neck inside diameter at that point, which will, not may, have an influence on the amount of constriction surrounding a seated bullet. And since it won’t be perfectly consistent from case to case, accuracy will, not may, suffer.
And, without a doubt, there’s going to be cartridge pressure changes, which can create velocity changes. A donut is not likely to create anything like a pressure spike similar to what an excessively thickened (overall) case neck can, but it can’t be a Good Thing no matter what.
Now. I can’t say this is always a symptom of aging cases (based on the “four firings in” idea I’ve been running with). I’ve seen donuts in new cases. However, in my experience with the brass I normally use, and, therefore, that which I have the most notes on, the formation of a donut seems to coincide at the same time I measure what I think is excessive case neck wall thickening. Again, though, I spent an afternoon at the loading bench with David Tubb trying to solve donut issues he was having after one firing on commonly known “good” brass. We solved them, and more in a bit.
There is a difference in the case wall tubing thickness at the case neck, case shoulder juncture. The neck walls are a consistent thickness — it’s a parallel cylinder (or they start off that way). At the shoulder wall thickness increases steadily in a taper as it goes down the case shoulder to then intersect with the case body walls.
There is diverse speculation about exactly what causes or creates the donut. My own experience suggests that there can be more than one factor or influence. But at the root of it is simply this difference in wall thicknesses. The difference has an influence in this area with respect to brass flow. Seems certain that there’s material movement forward from the case shoulder.
If that’s it, then the chamber dimensions (neck diameter and headspace) and cartridge case headspace play their parts. Same old: with respect to case headspace, it’s another reason to set back a shoulder the minimum amount needed for faultless function. Also old news: that’s going to be more for a repeater than a single-shot, and well more for a semi-auto.
I’ve seen it said that the expander ball or sizing button coming back up through a sized case neck “drags” the metal up with it, but also I know without a doubt that sizing without an expander means there’s a more pronounced donut. Checks I’m made sizing with and without an expander (using a neck-bushing-style die), show that an expander or, my preference, an expanding mandrel, reduces the donut influence. That, by the way, is from selecting bushings that produce the same case neck outside diameter with and without the inside neck sizing. I think the expander is just pushing it to the outside… But that’s good!
This one is pretty easy, after a little math at least. The most direct means is using a correctly sized reamer on a likewise correctly sized case neck, and that’s where the math comes in. The reamer should be the diameter of your sized neck inside diameter; that will pare away the donut without changing the case neck wall thickness. The idea is to get the donut without universally thinning the case neck walls, and the reason there is maintaining consistency. That, after all, is why we’re doing any sort of fixing on cases in the first place: get the same performance the maximum number of firings.
Another way, which is primarily preventative, is with an outside case neck turner, if its cutter has an angle or bevel (see photo for example). Turn down onto the case shoulder about 1/16 of an inch. Do this on new cases since that’s the only good time to turn case necks. This area is then “relieved” enough that the donut won’t form, or not for a while. In firing, this thinned area essentially relieves itself. I got this tip from Fred Sinclair eons ago and it’s the only thing I know of that heads off the donut. If you are worried about weakening a case in this area, don’t do it, but I can tell you that’s a moot worry. It’s very common practice among competitive Benchrest and NRA High Power Rifle long-range shooters. That’s how we came to a quick and permanent (well, for the short life of those cases) solution to David Tubb’s donut problems with a 6mm-.284.
Short aside note that’s being revisited from other articles I’ve done here, but the VERY BEST way to never worry about donuts is to never seat a bullet into this area! That is the reason the better (in my mind) cartridge designs feature long necks.