Understanding the "Dreaded Donut"

[Straightshooter1]

Since I generally FL size with custom dies, I will leave this to the neck sizers among us. Generally I try to have enough free bore so that doughnuts are not an issue.

Yup. Seating so none of the bearing surface contacts that area is a good solution, or . . . using an expander mandrel to expand the necks (especially with annealed brass) pretty much uniforms the inside of the neck so that things like donuts or tapered wall thickness or variations in wall thickness is not such a big deal.

 

[INTJ]

Great explanation, Boyd. Many thanks. I get it that neck-only sizing will in time lead to overly-tight cases on chambering. My question was more whether, with neck-only sizing, one would avoid the donut (at least before FL or body sizing to restore light chambering).

No one serious about long range accuracy neck-only sizes. All the LR BR competitors I shoot with and all the F-Class shooters I have come across FL size. A lot of these rifles have tight necked chambers that require turning. Donuts aren't much of an issue for us.

I have eight firings on my current batch of 6 BRA cases. I neck turned them slightly into the shoulder, and I have had zero issues with donuts. Part of that is because of how I turned the cases. Part is because my chambers are throated to allow the bearing surface of the bullet to be above the neck shoulder junction, so even if a donut were to develop it wouldn't affect anything.

A couple years ago I chambered a 33-28 Nosler elk rifle. Necking up the 28 Nosler brass to .338 pulled a little bit of shoulder into the neck area. That area was .006" larger than neck diameter. To fix this I simply turned the necks lightly, just enough to get rid of the thick area that was created by necking up. I haven't had any donut issues with this gun.

Several years ago I had a 300 WSM built on a M-70 CRF action. I had the smith use the reamer I had used on my first 300 WSM LRBR rifle. This chamber required I turn the necks. I haven't had any donut issues on this rifle either.

In fact, with all the rifles I have had to turn necks for, none have ever given me any donut issues.

And I quit neck only sizing 30+ years ago.......

 

[M-61]

creating sufficient friction between the body of the case and that part of the chamber that any stretching of the case back to the bolt face happens toward the back of the case.

A good reason to chamber, clean (not oil filmed) brass.
Oil on brass is best understood when you read how the Brits gave the final proofing of their actions in Lee Enfields.

You can also end up with lug galling issues from running tight brass, especially if your lugs are not lightly greased on their rear surfaces on a regular basis.

The lugs are probably the most neglected I have seen in many rifles. Bone dry.
I guess running a differential in a car with zero lube is a decent analogy.
Not saying what is the best lube but decades of using a wipe of wheel bearing grease works for me.