Neck turning runout question

[mikecr]

Mikecr: When forming my 20 P from 223 brass with a Redding bushing FL die I wound up with a donut that caused some initial grief in load testing.

You didn't create additional donut brass with downsizing.
The problem area(additional thickness) described is there at any cal.

Brand new cases taper in thickness from mouths thru webs. This is inherent to manufacture.
So neck-shoulder junction is slightly thicker than neck thickness near mouths.
Normally we would seat bullet bearing enough above neck-shoulder junction to clear this.
If you seat into it, or size it, you bring it into play.
These are choices, as reaming it clear is a common followup choice.

When neck sizing up, we're actually adding to the issue, both in thickness and in length of that deviation.
If not mitigated with preferred neck turning, bullets will need to be seated even shallower to clear it.

 

[charlesasmith]

mikecr: I am a little dense so need more explanation, if you don't mind. I used new IMI 223 brass and have to seat the 20 cal 39 gr Sierra past the neck shoulder junction due to zero freebore and long bullet shank. I used a bushing FL die that left a short unsized .224 cal area @ the neck/shoulder junction. When testing for bullet seating depth, I experienced a blown primer when going to 0.020 off jam. Finally found the thin donut that caused the problem and used the inside/outside neck reamer/turner to take it out. Since the neck/shoulder junction is thicker than the rest of the neck why not turn into that junction a few thousandths to reduce future neck thickening problems after multiple firings?

 

[mikecr]

You can turn it if you can get a good turning mandrel fit.
All I was suggesting earlier is that donut mitigation is normally not needed for downsizing of necks, where it often is for upsizing.
The zero freebore issue (that choice) is separate and different.