5 Attachment(s)
Another way to avoid a donut ?
I’ve been thinking about necking up some more 6.5-.284 Lapua brass for use in a .284 Winchester, and ways of reducing the occurrence of donuts. The same would apply to other similar forming where the case neck is being taken up in calibre. Not that bullets will necessarily be seating deep enough to where the shank will contact the problem area, but you never know initially just what engagement or jump a given bullet is going to prefer and so how deeply it will end up being seated. I figure it is best to prepare the cases to accommodate all eventualities and try to remove any donut and have the inside of the necks as uniform in diameter as possible - top to bottom.
There are two methods commonly used to fix the donut issue when taking necks up to a larger calibre: use an inside neck reamer to remove them, or turn off the excess brass with outside neck turning gear after expanding them to the new calibre.
Looking at how donuts are formed using diagrams that show the cross-sectional profile of the top part of the cases: running an expander through the neck should give B shown below right from the parent case A in the original calibre, where a portion of thicker shoulder brass is moved around into the bottom of the necks.
Attachment 128427
On firing, the brass in the neck conforms to the chamber profile, and a donut develops internally, shown in C.
Attachment 128428
One strategy to reduce this is to take the expanded brass at B and turn the cases, removing the excess thickness at the base of the necks while perhaps also reducing the shoulder thickness in the process to much the same as that of the neck, as shown below in D. The case is then fire formed to E, hopefully now without a donut developing.
Attachment 128429
The outside neck turning gear will require a cutting bit ground for the appropriate shoulder angle. Most reloaders seem to take this indirect route of outside turning to prevent donuts rather than the alternative of reaming them out.
However I have found that B is not a true representation of what happens when the necks have an expander run through them. My observation is that an expander is not effective in pushing ALL the excess brass to the outside of the cases and instead leaves partial donuts, as pictured in F, unavailable for removal solely with outside turning.
Attachment 128430
So my question is: has anyone here instead gone down the path of thinning the shoulders of the cases in the
original calibre down to the same thickness as the necks prior to expanding, as a method for reducing the occurrence of donuts? That is from A to G below
Attachment 128431
I’ve not tried this but am thinking that G may be more likely lead to donut-free cases?
For the .284Win this will mean thinning the shoulders of the 6.5-.284 brass prior to expanding the necks. The necks can still be uniformed again for thickness once at .284” and at the same time the shoulders checked and re-cut as necessary. Extra work, as the cases will be turned twice, but hopefully no donut.
