See the end of the carbide .260 mandrel on the K&M neck turner. Those flutes are a donut cutter. If you have or get donuts at the base of the neck, this cuts them out from the inside. Problem solved.
NormB
As Leeper pointed out "all" brass is thicker at the base of the neck than at the mouth. As Jerry pointed out this extra brass, at the base of the neck, starts on the outside and is forced to the inside of the neck when fired. This extra brass is what causes that dreaded "donut" and is present on both the new case and the fired case. Uneven neck tension from case mouth to base of the neck is the result.
Outside neck turning a new case only gets rid of part of the excess brass when using standard neck turning procedure. With the K&M tool, shown in the picture, standard procedure is to run the K&M Expander into the neck so it will accept the standard pilot and turn. This still leaves the mouth thinner than the base, as shown in the picture, unless you turn the necks thin enough too clean them up. This leaves them too thin at the base, when you neck up, as suggested by Leeper and causes excessive neck clearance in standard chambers. Inside neck turning a fired case results in the same problem, only in reverse.
Experience in sizing up a case for my 7mm-25 WSSM and sizing down cases for my 20EXTREME (222Rem) and 6.5EXTREME (300RCM) has taught me a few things:
1. That it is possible to get a uniform neck, when necking
up, only if you have a thick enough neck to start with as
necks tend to get thinner when stretching.
2. It is easier to get uniform necks by necking down as
they thicken which allows more brass to be turned
while still maintaining proper neck thickness.
3. It is better to turn the necks, slightly below desired neck
thickness, just to the base before sizing down. This way
the necks will maintain even thickness through more
firing/sizing cycles.
4. I use the K&M turning tools to get a uniform neck
from mouth to base. First I use the Expander but only
run it into the neck about 3/4 of the way, or just
enough to expand the mouth enough to leave about
1/2 of the "donut" on the outside and the other 1/2 on the inside.
I then turn using the carbide mandrel, shown in the
picture, cutting the excess brass from both inside and
out. If the neck is thick enough it will be totally cleaned
up and uniform. Using this method works best, I think,
because of the difference in spring back from the case
mouth to base when using the expander past the base.
5. Once uniform neck thickness is achieved it doesn't
matter whether you use collet die, expander die or
bushing die you will have uniform neck tension.
6. The amount of neck tension can be adjusted by using
neck bushing but will only be the same provided all
necks are the same thickness from mouth to base. If
they are not then it is best to use a custom expander
ball or the collet die as Jerry suggests.
I have attached a drawing of formed brass for my 6.5 EXTREME with a Berger 140VLD seated to the lands. The necks are a uniform 0.0160" down well past the base. As you can see it is properly designed for the 140 VLD to fit in the clip using a Long Magnum Action. Three fit and feed perfectly from a Rem 700 clip. It will drive this bullet to over 3000fps while maintaining low enough pressure that the brass has Zero Stretch at the WEB. Should get 6-7 Full Length Sizing's before having to trim.
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Here are the first test targets, shot from the bi-pod and rear bag, after load development. The top two bullets on the 400-500 Meter target were shot by a 300 Win Mag, with Berger 210 Vld's, for wind-age comparison.
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No question which 6.5 I would choose.
Which considering how badly Remington dropped the ball where
