RELOADERS CORNER: Four Firings In: Final

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Yikes. Gremlins. Case neck “donuts” are a common development in an aging cartridge case, and it’s often unknown. Read this and know! MORE

case neck donut
Here one is! Or was.

Glen Zediker

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.

Culprits
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!

case neck donut neck turning
This helps! Turning a tiny bit off the start of the shoulder gives some relief in this area and holds off the donut for at least a while.
neck turning cutter angle
The neck turner, however, has to be configured to allow for this. Note the bevel on this cutter.

Fixing It
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.

neck reamer
This is a “special” reamer, meaning ordered to a custom and specific size. Choose carefully, and it’s an easy fix.

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.

Glen’s books, Handloading For Competition and Top-Grade Ammo, are available at Midsouth HERE. For more information about other books by Glen, visit ZedikerPublishing.com

Glen’s newest book, America’s Gun: The Practical AR15. Check it out HERE

par15

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16 thoughts on “RELOADERS CORNER: Four Firings In: Final”

  1. “…better cartridge designs feature long necks” you must really like the 222, however, you never mention it.

    1. I have mentioned .222 Rem. I like it fine. I built one for reduced distance High Power courses and it’s the smallest groups I’ve seen from an AR platform rifle. It wasn’t good for much else but it wasn’t supposed to be. Never fired it beyond 100 yards. It outshoots .223 Rem. by a good ways. Usually when I think of a long necked case it’s a PPC.

  2. Due to the taper of case walls, oversizing causes brass to migrate from the base to the neck. That’s why it is necessary for most people to trim cases. Size them too much and eventually, you’ll get the donut and longer cases. I only bump the shoulders back about .003″, never have to trim cases, and don’t get donuts because the case brass doesn’t migrate.

    1. It would have nice to have a before photo, rather than or along with, the after picture, so one could see what the problem looks like..

      1. That is true. It’s honestly a very small bit, the photo just looks more dramatic because of the contrast where the reamer made contact.

    2. John, on the adage that the only stupid question is the one unasked, would you explain exactly what you mean by “only bumping the shoulders back about .003”? I am getting into reloading with standard equipment after years of Lyman 310 experience. I find I have to fl resize almost to the neck/shoulder juncture to get the case to chamber. I intend to neck size after fl sizing for my bolt guns but of course will fl size for my AR 10. I am loading for accuracy, not velocity. All comments will be heartily appreciated.

    3. John, after years of loading via a 310 tool, I am currently using conventional equipment. When I fl size bottleneck cases I find I have to size almost to the neck/case wall juncture to get the case to chamber. Would you please explain what “I only bump the shoulders back about .003, never have to trim cases, and don’t get donuts because the case brass doesn’t migrate” means? Thanks for any help you care to give.

  3. I’ll try again, apparently the reply function doesn’t work. John, would you please explain what your comment on bumping back shoulders about .003 and avoiding donuts and trimming means? I am loading with conventional equipment now, but loaded with a 310 tool for many years. Or anybody else, if John doesn’t see this. Thanks.

    1. If you FL size a case as most instructions tell you to, you end up having to trim cases every few reloads. That’s because you’re oversizing them, and forcing the brass to migrate to the neck area. Minimal sizing, where you only move the shoulder enough to ensure chambering, does NOT cause the brass to migrate any appreciable amount, thus no trimming. That also eliminates the formation of the donut. I have a couple thou of 277 Wolverine wildcat cases. Most have been reloaded 4 or 5 times, none have ever been trimmed after the initial forming trim, no donuts, etc. Shoulders get bumped back .003″ only. The key is using the Hornady Headspace tool to adjust the sizer. Should be able to do that on the 310 if the die is adjustable at all.

      1. Please consider one more question, John. I am currently using mostly conventional RCBS dies and a Redding 7 turret press. Cases are once-fired range pickups. I fl resize to the neck/shoulder juncture to enable the case to chamber. Are you saying many folks resize to the point of moving the shoulder back from there? As a minimalist in many things, I ask, why would they do that? And finally (Gee, that’s more than one question,isn’t it?), you are saying you stop sizing the neck .003 before the neck/shoulder juncture? FINALLY, what do you think of collet neck- sizing for cases intended for a bolt gun? (Geez, another question!) Thanks, Gerry.

        1. OK, first terminology needs defined. Cartridge headspace is the distance from the base to the midpoint of the shoulder of the fired case. (If you set the sizer according to the die maker, you typically end up with excessive headspace of anywhere from .007-.013″ and that is way far excessive.) Call that your base length for resizing. Let’s say it is 1.53″. You obtain that by measuring with the Hornady Headspace gauge. To bump size those fired cases, you run them into the sizer and adjust it down until the value on the gauge is 1.527″, or .003″ LESS THAN the initial fired length of 1.530″. All you’ve done is push the shoulder back by .003″. That prevents the body of the case from migrating, so there is no stretch which ultimately would require trimming the neck. Basically, the shoulder just occillates between the fired length and the bump sized length as measured at the mid point of the shoulder. You will still have to trim OAL to make sure you don’t exceed the base to the mouth of the case maximum, but most of us just trim the neck to the minimum trim length, and never have to do so again for the life of the case. Btw, chamber headspace is the distance from the bolt face to the midpoint of the chamber shoulder, but it is fixed when the chamber is reamed. Cartridge headspace can change because the brass case is maleable, and can change due to either firm forming or sizing. .003″ bump is good for ARs, and .002″ is good for bolt rifles. Some guys like to go a touch more, but I’ve found those values to work for all my rifles.

          1. Good points. Most seem to think that no setback is better still. I have to get some clearance, and it’s usually that 0.003 figure, or 0.004. BUT. Donuts still can form regardless. I honestly have seen them in new cases. Semi-autos need the shoulder bump for safe function. And, just a technical point, but the point on the case shoulder where headspace indexes from is the datum line, which is a point that corresponds with a diameter. It may or may not be at the midpoint. There’s only 5 datum figures in use for virtually all bottleneck cases. For .223 Rem. for instance, it’s 0.330 inches.

      2. Please consider one more question, John. I am currently using mostly conventional RCBS dies and a Redding 7 turret press. Cases are once-fired range pickups. I fl resize to the neck/shoulder juncture to enable the case to chamber. Are you saying many folks resize to the point of moving the shoulder back from there? As a minimalist in many things, I ask, why would they do that? And, (Gee, that’s more than one question,isn’t it?), you are saying you stop sizing the neck .003 before the neck/shoulder juncture? FINALLY, what do you think of collet neck- sizing for cases intended for a bolt gun? (Geez, another question!) Thanks, Gerry.

  4. Collet neck sizing works great for 222 bolt guns. Whenever the bolt doesn’t close easily I use a Redding body die to reset headspace. Because I shoot light heads (35-40gr) & light loads for accuracy & use good cases, Lapua & Norma, headspace correction is not much of a problem. Usually once a year I anneal, trim, & body size, there may have been easily 10 or more firings I don’t keep count. This is a minimalist management scheme that works for me.

    1. John Hull and BK, thanks much. I understand now. Knowledge is indeed power. BK, I encounter a number of makers.Would you please rate the top few case makers as to accuracy, what to look for, etc.? How about the worst say five makers? Now, that’s info I would cherish. Really,really. FINALLY, what do you think of military LC cases? As to Lapua and Norma, I’ll have to buy new, you don’t see them much here. Thanks, everybody.

      1. Remember that I load 222 so I can’t comment on LC & other military brass. I’m confident the case manufacture matters very little for accuracy, matters much for durability though, according to how you load & shoot. What does matter is case volume & neck tension consistency. I use prepped weight as a proxy for volume. I initially prep all my cases: body size, neck turn, primer ream, weigh/sort, mark, & annually anneal. I then load by marked case weight lots, at this point case brand or lot makes little difference, at least the way I shoot in bolt rifles. For 222, I find Lapua the best quality out of the box case with very little weight variation, Norma are good, just had significantly more weight variation. Frontier cases are the worst I’ve ever seen, Win Super-X & Rem in the median. These issues are production/quality control issues & will vary by lot. But when I discard a lot due to neck splits or loose primers, it’s replaced with Lapua.

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