Thread: Concentricity

Based on some discussion in the "here and now" thread.

Concentric, at least the dictionary definition, means to have the same center.

So you could look at reloading concentricity as having inner and outer surfaces on a case neck spaced evenly apart - in other words having consistent neck thickness.

My understanding is that concentricity is more often used to talk about the neck circle and case body circle both having the same center, i.e. The neck not being skew or offset, also called runout.

In most chambers there is a small gap between the chamber wall and the outside surface of the case neck. Once you fire the bullet, the case expands fit tight in the chamber, and then springs back slightly (your "fireformed" neck diameter). With competition or custom chambers, that gap can be very small.

The fireformed neck diameter is often large enough to not grip a projectile, so you have to squeeze the neck down to a diameter small enough to hold the projectile, now we are starting to talk about neck tension.

Obviously if the neck is thicker on one side than the other, concentricity goes out the window, which is why some folks do neck turning. If the neck is an even thickness, the chances of consistent neck tension and minimal runout are higher. The type of sizing die you use can also affect concentricity. Dies that have a good reputation for minimising runout are bushing dies and especially collet dies.

@kiwi39 @Maca49 @veitnamcam interested to hear your comments and theories

bushing dies, good brass, good rifle, never worry about it

If your fired brass has significant run out forget trying to straighten your reloads.

Gauges that measure loaded rounds at the neck and ogive while holding on the body walls are the best IMHO, ones that hold the base and tip of projectile don't tell you if it is angular misalignment or eccentric neck that is giving the runout.

Actual eccentricity of the neck compared to case is rare if chamber is sweet and dies used correctly, which in the case of un turned brass means the bullet will be offset by the variation in neck thickness.

More common is a concentric neck with a bullet seated slightly at an angle, which if minor is easy enough to correct.

I made myself a jig to measure concentricity and graded my reloads by run out and shot some groups with the worst and some groups with the best, any difference was lost in the noise of my shooting ability so now I just load and shoot.....but this is for hunting. If I was shooting competitively I would take every possible measure to ensure absolute consistency at every step of the reloading operation.

Originally Posted by veitnamcam

If your fired brass has significant run out forget trying to straighten your reloads.

Gauges that measure loaded rounds at the neck and ogive while holding on the body walls are the best IMHO, ones that hold the base and tip of projectile don't tell you if it is angular misalignment or eccentric neck that is giving the runout.

Actual eccentricity of the neck compared to case is rare if chamber is sweet and dies used correctly, which in the case of un turned brass means the bullet will be offset by the variation in neck thickness.

More common is a concentric neck with a bullet seated slightly at an angle, which if minor is easy enough to correct.

I made myself a jig to measure concentricity and graded my reloads by run out and shot some groups with the worst and some groups with the best, any difference was lost in the noise of my shooting ability so now I just load and shoot.....but this is for hunting. If I was shooting competitively I would take every possible measure to ensure absolute consistency at every step of the reloading operation.

I concur vc
I shot some groups at 600 with some dead straight ammo and some with 10 thou run out.
The straight group was 3 inches and the bent 3.5 inch so yep the concentricity guage is now obsolete!

Well I've only just got the gear , so we'll see how it goes.

My aim is increased consistency by making the neck as consistent as possible without taking too much off.

I neck size with the Forster bush n bump and so what I should see through this is a more consistent seat due to a more consistent neck wall thickness

The first tests are showing smaller groups than I got before with my 223. Have yet to turn the first tray of cases for the 243.

Work in progress

Tim

Originally Posted by hamsav

I wonder if the decision to go with a 14' twist was a good one ?

Never, I'm afraid

@andyanimal31 your 10 thou runout - was that on the neck or the projectile ogive ?

VC, when you say use dies correctly to minimise eccentricity can you explain some more pls

O

Originally Posted by kiwi39

Well I've only just got the gear , so we'll see how it goes.

My aim is increased consistency by making the neck as consistent as possible without taking too much off.

I neck size with the Forster bush n bump and so what I should see through this is a more consistent seat due to a more consistent neck wall thickness

The first tests are showing smaller groups than I got before with my 223. Have yet to turn the first tray of cases for the 243.

Work in progress

Tim

I'll be keen to know if there is a difference, many small adjustments may help accuracy, it wouldn't help my ability, I would still be the weakess link!

Originally Posted by ebf

@andyanimal31 your 10 thou runout - was that on the neck or the projectile ogive ?

VC, when you say use dies correctly to minimise eccentricity can you explain some more pls

Well in the case of a neck die not trying to size the neck too far down and in the case of a full length just ensuring the decapping rod/expander is not bent and the inside of neck is well cleaned and lubed, and the outside of case is only lightly lubed.

And that the seater is set at the correct height, particularly in the case of a seat and crimp die.

Originally Posted by ebf

@andyanimal31 your 10 thou runout - was that on the neck or the projectile ogive ?

VC, when you say use dies correctly to minimise eccentricity can you explain some more pls

I just measure between the olgive and the neck with the hornady gauge.
I occasionally check now but usually 0 to 3 thou run out on my 6.5 using bushing neck dies which is close enough for me.
The one thing i have found with reloading is the more you know the less you know!

Originally Posted by andyanimal31

The one thing i have found with reloading is the more you know the less you know!

Yup +100

Had a fascinating conversation with one of the top f-open shooters in the country yesterday. His other hobby is benchrest... These guys are loading for 5 shots thru one hole type accuracy. The chamber tolerances are bit tighter than your average factory hunting rifle.

During NRA nationals this year we had the Ausie f teams over, and they were awesome with info sharing. The level of reloading for competition is closer to benchrest. But the results are there, they shoot perfect water lines at massive distances, so the only thing you are fighting is wind.

Some of you guys must have to much spare time?
I have shot some ridiculously small groups at 1000 yards in the past just using good dies and good brass, my reloading technique is very "agricultural" basically self taught,
and I'm the weakest link in the equation, so I always thought why bother with this stuff?
I will follow this thread with interest I might take something away from you more knowledgeable guys.

Who knows which one of the thousand variables causes greater accuracy

All I know is that reloading to this level is a fantastic way to keep people (no names will be mentioned) with OCD tendencies calm and occupied

Not talking about occasional tight groups here. This is consistent, on-demand, every time type results with 10 shot groups.

I've got some of my "old style" target rounds left, then some rounds with turned necks, and lastly rounds where I went completely anal and measured cases, projectiles etc to the n-th degree. I'll shoot them all over the chrony in round-robin fashion and it should be interesting to see the SD differences between the batches...

As odd as this may sound @BRADS , I have too little time to spare.

Since I dont have the time to go hunting as much as I want, I enjoy getting the most out of the gear I've got , so that when I do go, I know very well what I'm dealing with.

I'm also lucky enough to have the space to have my own range So I can pretty much step out of the workshop and see the results on paper as it were. (Watch this space for my "new range" project this year)

neck turning is actually a pretty good investment in terms of reward for time spent - and I only ever spend time in the workshop when I dont have anything else I could be doing outside ... so its not time wasted for me.

And anyways, it keeps @ebf's mate ( dont know the guy but I'm sure he'll introduce us ) so it must be a good thing .

I'll put results / experiences / methods etc up here as they happen.

T

Don't you mean 60M  Andy!