what causes the donut ?

[jhalcott]

necking up or necking down? I have reformed cases to smaller AND larger diameter bullets. Some times I have to ream the necks out , other times turning them to reduce thickness is all that's needed. A friend told me necking UP( .223 to .257) will cause the donut. Another guy swears going from .257 to 6mm does it!

[jj850]

It is when you start bringing thicker brass from the shoulder into the base of the neck. Or pushing the larger diameter of the shoulder in to the smaller diameter of the neck, still the same amount of brass it has to go somewhere so it gets thicker and makes a donut. Necking up can pull the shoulder into the new larger diameter neck. Necking down usually makes the neck thicker for it's full length.

[country gent]

donuts can be formed in several ways. One is as mentioned shoulder brass being formed into the neck. Not reaming completely thru the neck, and not turning far enough down or a cutter that isnt a complete match to the brass. Neck reaming is the easiest way to remove or not form donuts.

[swheeler]

donuts can be formed in several ways. One is as mentioned shoulder brass being formed into the neck. Not reaming completely thru the neck, and not turning far enough down or a cutter that isnt a complete match to the brass. Neck reaming is the easiest way to remove or not form donuts.

Good answer. I've heard that LEO's do not remove them?

[rosewood]

When necking up, does the donut form on the inside or outside of the neck? If on inside, what tool is available to remove it?

Thanks,

Rosewood

[country gent]

If the neck is supported in a die so it can spring around the reamer when reamed inside reaming will remove it. Or a neck turner that matches the shoulder neck angle correctly will remove them. On my one wildcat round ( a 300 wsm necked down to 6.5 mm and shortened) I neck ream in the last sizing die and then neck turn for concentricity and wall thickness. More work yes but neck wall thickness dosnt vary by more than .0002 around the neck when done. This cartridge was desighned for a 1000yd round so accuracy is upmost.
When neck reaming I like to have the case in a die supporting the neck and holding it from springing or expanding around the reamer. If your going to have to thin necks on reformed brass then a chamber cast to show neck dia is very helpfull. Your doing the work anyways so making a "fitted " neck isnt anymore work. Simply turn the necks Loaded dia .001 under the chambers dia.

[lotech]

I'm no expert, but have found that by turning to the extreme end of the neck, there will be no "donut" formed. I've missed the mark by just a fraction on occasion, and the donut will appear after firing.

I won't argue neck turning vs. neck reaming; some find one or both necessary and beneficial. I've never seen a need for reaming if a case is neck-turned properly. However, there are exceptions to everything.

[rondog]

Mmmmm.....