There at least 2 cases where it can be very important when shooting fixed ammo.

1. If the brass has uneven case walls.

2. If a chamber has to small a neck, as in match chambers that are made small to require neck turning or reaming, to guaranty neck will be concentric and in the case of paper patch chambers that are made to hold bore diameter bullet and you want to shoot a groove size bullet. It must be done the hold and chamber the groove size, GG bullet.

Frank

"1. If the brass has uneven case walls."


Now to see how to ask a question. If one has some cases that have uneven walls and want to turn the inside of the neck, how does the cutter remove the "thick" part rather than follow the hole. I have two Pacific case trimmers and a Lyman Pro Trimmer. Neither exhibits the ability to hold the case to allow a neck reamer to even out a case neck.

Reading about the bushing method or the neck collett method of neck sizing, they are touted to actually even up the case neck walls. Opinion or experience?

40 Rod, I have obtained A RW breech seater and have been talking to a member here and Tom of Accurate Molds. We may have come up with a BS bullet for me.

I outside turn necks also, but mainly because it doesn't matter where you stop at, it wont leave a ridge inside that the bullet will stop against when seating fixed ammo.
And my case trimmer can be set to a specified thickness, so with the inside mandrel it wont take metal off thin areas, just thicker walls.
As previously mentioned, brass for breech seated bullets doesn't need any neck work.

I've always inside neck reamed when I was necking down a case and the neck walls became too thick. It is usually necessary to anneal the cases after doing this.

He is referring to workholding. The case would be held in a dummy chamber cut with the same tool as the chamber. The reamer must be guided by a very close tolerance hole cut in the same operation for concentricity. The reamer should be very short for rigidity. Such a set up should yield concentricity of .002" in brass I would guess.

tdmidget wrote on Sep 2^nd, 2018 at 7:45pm:


I don't turn the ID, so I'd never make such a jig. I also have almost exclusively old guns, so couldn't find the original reamer either.
I prefer turning OD of case necks and my K&M tool uses mandrels to fit the inside neck and easily turns the outside consistently the same thickness. I have made some spare mandrels for odd calibers I want to turn or thin.

marlinguy wrote on Sep 2^nd, 2018 at 6:37pm:

For an idea of how it works, take a look at the LE Wilson case trimmer. You can think of their case holder as a section of 'stub chamber'. It holds different cases based on the diameter and taper of the different holders. They have an inside neck reaming cutter that can be held in alignment with a case, but I believe even they recommend cleaning up by outside turning so the inside reamer doesn't just follow an offset neck. 

I'd go along with your thought and do outside turning only for my odds and ends. I believe inside reaming is doable, but it's much tougher, for me anyway, to arrive at a consistent and concentric neck thickness for one let alone a batch of cases.