Necking up cases and donuts

[superx10]

Hi
I am new to tight necks and necking up cases, I have just started shooting a 284 win in F open and naturally purchased Lapua 6.5 * 284 necked them up and turned the necks to suit a .312 diameter, on final inspection it appears that a donut has formed on the inside and at the intersection point of neck and champher . I have 7mm cutter that suits my K&M neck turner but was hoping for some information from some people with more experience, any input would be appreciated as I am sure with the amount of f open shooters who shoot the 284 win this has been dealt with often.
Thanks in advance

[AlexE]

Like you've suggested, I use the k&m cutter pilot. No donuts. Used the same setup to create 260 brass from. 243 before good 260 brass became available

[williada]

Richard Utting sums it up pretty well. https://www.youtube.com/watch?v=EyJ9KkE2pVc

[Bigtravoz]

Might sound silly but wouldn’t reaming the internal of the neck solve the issue?

[GSells]

[Bigtravoz]