Please help me understand the "doughnut"

[bluto77]

Hello all. I'm new to the forum, but not new to shooting, yet somewhat new'ish to reloading (been doing it for nearly 2 years). I feel like one thing I'm good at is "knowing what I don't know", and I've gotten pretty good at using shooting forum search functions to figure those things out. And searching for information on one topic almost always leads to more questions about related topics. I preface my question with all of this, because out seems like the good old search button should help me answer it, but I can't seem to find a basic explanation.

What is the "doughnut"?

I'm in the process of having a F T/R rifle built chambered in 308 and shooting the 185 Juggernauts. I was doing some searches on freebore/seating depths/optimum velocity/neck turning and kept coming across explanations in which people talked about having enough freebore to not have the bullet seated in "the doughnut". I can reason that it refers to a certain part part of the neck, but wasn't sure why you would or would not want your bullet seated there. Any further explanation would be appreciated.

By the way, I'll be using a neck turn reamer (not sure if saying that right) with .168 freebore as I would like the option of shooting the 210 - 215's as well as the 185 Juggs. This will be my first rifle that requires neck turning, but I'm ready to go to that next level. I'm ultimately trying to figure out a target velocity for the Jugs, and how to get there while minimizing pressures by taking advantage of longer seating depths, thus giving more case capacity. Running a Broughton 32" 11tw barrel, Stolle Panda from Kelbly, and PR&T stock.

All things considered....please tell me about the doughnut.

 

[Larryh128]

A doughnut is basically a lump or bulge in the neck of the case on the ID of the neck. There are several causes but a lot of them are caused by fireforming cases. I tryed to find a decent desciption on this forum but it eluded me. I'm sure there is an article here about them. They are easy to detect and can be removed the the proper size reamer. A bent pice of wire dragged over the case neck interior will detect them easily.

 

[Nomad47]

My understanding is that there are two ways to get a doughnut. Both involve thicker shoulder material becoming neck material that is thinner.
The first is when you form a case like the 6SLR from 243 brass like Lapua which has thicker shoulders than necks. When the 30 degree shoulder is formed from a 20 degree shoulder, shoulder material becomes lower part of the neck.
The second is from normal brass flow that occurs as trimming is needed. The thicker shoulder material migrates up into the neck.

 

[BoydAllen]

IMO, particularly when you are putting together a rifle for a specific bullet, the best way to deal with this issue is to adjust your freebore length so that the full diameter part of the bullet does not extend into the part of the case neck right next to the shoulder. As you fire and size your cases, depending on how much your die reduces the diameter of fired case bodies, and how much you bump shoulders back, brass will be forced up from the shoulder to the neck, thickening the neck where it meets the shoulder. On bumping, it takes more than one firing for the shoulder to head dimension to reach its maximum, so I usually neck size and fire a case until its "headspace" hits its apparent maximum, (making sure that the back of my bolt's locking lugs are properly greased), and use it for a reference for setting bump. There is no point in creating more clearance than needed. When setting your bump to say .001 you may run into issues of consistency of bump that have some cases with tighter bolt close than others. Fixing this, is a whole separate topic, but I will say that if you have differences in bolt close within groups, your groups will open up. If I do not have the opportunity to do this, I set my die so that cases' shoulders are where they were after their first firing. Any time that I am measuring a fired case, I either remove, or reseat the fired primer. Getting back to your die, the closer the match is between it and your chamber, the less brass movement up the case and trimming you will have to deal with.

 

[fm1947]

Also when turning necks if you don't get into the shoulder correctly it will cause a doughnut

 

[stgw77]

I also use .168" FB w. 185 Juggernauts, and you will not have any issues with potential doughnut using this freebore, only a little of bullet dia. stays in neck (.140"-.150") when touching lands.

 

[lpreddick]

a bushing neck die can form a donut immediately if sizing "too much" diameter of the neck as making 22 BR from 6 BR lapua. doing this in stages might avoid the donut, but not always. in such long neck cases it is not likely the bullet's base will go through the donut...a definite thing to avoid. an expander mandrel might be able to "press out" an early donut but once established, reaming is the only way. i tried pressing out a donut by seating a flat based bullet through it and due to it's hardness, the donut compressed the bullet's jacket.

 

[XTR]

I also use .168" FB w. 185 Juggernauts, and you will not have any issues with potential doughnut using this freebore, only a little of bullet dia. stays in neck (.140"-.150") when touching lands.

If you are planning to use a PT&G 2013 FTR reamer with a .168 freebore you'd need a NYC bagel in there to have problems. As stated above, the doughnut forms back at the neck shoulder junction. It is a problem if you are seating deep in the neck and you are getting thick brass with unpredictable or inconsistent tension gripping the bullet. If you are seating the bullets well up in the neck (like the 185s would be with the above mentioned reamer) you are never anywhere near where this thickening of the brass occurs.

 

[gotcha]

I agree with FTR. Keep in mind donuts can be felt during the seating process. If your seating stroke is slow and consistent then suddenly meets with increased resistance at the bottom of the press stroke you've probably got a donut. The type of dies you use can also alleviate or compound the problem should this happen to you.

 

[bluto77]

I also use .168" FB w. 185 Juggernauts, and you will not have any issues with potential doughnut using this freebore, only a little of bullet dia. stays in neck (.140"-.150") when touching lands.

If you are planning to use a PT&G 2013 FTR reamer with a .168 freebore you'd need a NYC bagel in there to have problems. As stated above, the doughnut forms back at the neck shoulder junction. It is a problem if you are seating deep in the neck and you are getting thick brass with unpredictable or inconsistent tension gripping the bullet. If you are seating the bullets well up in the neck (like the 185s would be with the above mentioned reamer) you are never anywhere near where this thickening of the brass occurs.

Perfect. This is exactly what I was looking for. I had an idea of what the doughnut was, but I wasn't sure why we didn't want to have the base of the bullet seated into it. Through my searches, I had read about cutting a window in the brass to make sure you weren't in the doughnut, and having freebore cut accordingly when building the rifle, along with a few other approaches, but no explanation of the doughnut itself. I am now hungry for a warm glazed donut and glass of milk to dip it in.

I'm not planning on jamming the 185 Juggs, as based on everything I've read, there is no need to do so. But it sounds like I should still have enough room with the. 168 FB to "stay out of the donut".

 

[Brians356]

donuts can be felt during the seating process.

They can also be felt using a pin gauge. I have a complete set of pin gauges, in .001" increments, from 0.169" through 0.500". These are "minus" or "go" gauges, in that they are actually 0.0002" smaller than their nominal sizes.

Anyway, I can take either a fired or sized case, and find the fattest gauge that will enter the case mouth. If there is a "doughnut" the gauge will either be obstructed by it, or feel a lot tighter, when trying to insert the gauge past the neck/shoulder junction.

Once you get a set of pin gauges, you'll find all kinds of uses for them (not just for guns and ammo) and wonder how you ever made do without them.

 

[stgw77]

Even if you are going to seat Juggernauts off lands, you wont be able to reach doughnut... 308WIN neck neck is approx .300" long, so you would need to seat them .150" off lands to reach dougnut... too big jump in my opinion. I found Juggernauts to be a very forgiving bullet, it works for me equaly well if 20 thou jammed or 20 thou off lands. The only issue you might have is with throat erosion after many rounds being fired. I also tried Bergers 210 LRBT using this freebore and works very well.

 

[lpreddick]

i have felt donuts by taking a fired case, brush out the powder residue and push a flat base bullet into the neck. a really tight neck won't let the bullet pass, but if the neck has expanded a few thous, the flat base will stop just at the donut.