question on case concentricity

[JustinP]

Hi guys

I have a batch of Norma 6XC cases that I received just a little while back - I think they are the 2006 lot.

On measuring case concentricity at body and neck, using a sinclair concentricity gauge, I've noted the following:

New cases/unfired/unsized:
Body runout average: .0009",great)
Neck runout average: .0021"

New cases/unfired/FL sized:
Body runout average: .0006"
Neck runout average: .0019"

New cases/unfired/FL sized, then neck turned to .013"
Body runout average: .0006
Neck runout average: .0023" increased??

Questions:
Neck wall thickness appears on my digital ball mic to be much better after turning, so why has the concentricity decreased after turning?

Is this sort of neck concentricity acceptable for new cases? The worst one I've had in the batch has been around .004", even after sizing.

FL die is a neil jones custom threaded bushing style
Turner is a Nielsen 'pumpkin'
neck wall thickness appears very uniform after turning

Feel free to tell me if I'm just being unduly fussy.

Cheers

Justin

 

[robbor]

Im guessing the neck was not even when sized, so when you turned the neck you induced a variation in contricity by evening the thickness up but not necessarly in relation to the body, you may have also cause some while neck turning but not from the cutter. FL resize the brass again and see if it is reduced.

 

[JustinP]

Thanks very much for the reply. That sounds like an entirely plausible explanation. I thought somehow that the turning had contributed to the issue. I'll run the turned brass through the FL sizer again and see what I get.

Need to get those necks straight, otherwise any bullet seating issues will only compound the runout issue...

 

[mikecr]

I don't understand the concern. And why any FL sizing prior to firing several times?
All that matters before fireforming is THICKNESS variance. You took care of it with turning.

Any RUNOUT before fireforming, is meaningless.
Much of it will go away after formed in your nice round chamber. It will return down the road with FL sizing if there is alot of thickness variance in the brass. So I don't bother turning/prepping/using such brass, and I don't FL size until I have to.

Your probably in good shape. Don't sweat it.

 

[JustinP]

Mike

I FL sized the batch I'm working on as some of them were a bit dinged up from transit and I wanted to straighten them out, particularly the necks, prior to turning them. I only have an FL bushing sizer for this caliber, so that's what I went with and what I'll be using to resize after each firing as a result. I really only put a batch over the concentricity gauge for interest's sake, to see what they were like prior to fireforming, after which I intend to check them again.

Putting them through the sizer improved the case body runout but the turning has simply induced a slightly greater neck runout variance - Rob's post makes sense to me. I think this will clear up after fireforming and I'm not in a lather just yet.

Probably just too much time spent at the reloading bench prior to my initial post.... maybe it should be called 'benchfever' or something.