The Misleading Case of Expander Balls and Neck Concentricity

Don’t believe the Hype!

Like many of us, I am always looking for opportunities to make better, more consistent ammunition. In many cases, while we are getting started, this often means reading and trying any and all suggestions on the internet. After trying this for a while, we realise the futility of believing everything you read on the internet, but I digress. One such subject of concern was that of the expander ball, found in most full length resizing dies.

Expander Balls – Function

In short, the purpose of the expander ball is to, surprising enough, expand the neck of a case again after it has been sized down are part of the resizing dies initial function. That is to say, when you run a piece of brass into a die, firstly, the expander ball has to pass through the neck, then the neck, shoulder and body is sized down by the steel internals of the die, then on the way back out, the expander ball again passes through the neck, resulting in a consistent (between different brands of brass, neck thicknesses etc) ID for the bullet to be seated into. Without this process, each piece of brass passing through the die, should there be any variation in brass thickness, will still end up with a consistent Neck Tension, once seated with a projectile¹.

And make no mistake, there are certain (many in fact) situations where you really, really need to smooth out the necks on the brass, dented necks being just one example.

Improved Concentricity

However, there was once comment that piqued my interest. That was, removing the expander ball could also increase concentricity (reduce runout), as well as remove this host of other issues that an expander ball could introduce into the case (pulling a bit of metal out through the sized brass, causing runout, expansion at the shoulder and so on. This could be demonstrated by simply running a piece of brass through a die, checking on a concentricity gauge, then running another (or the same) piece of brass through again, this time without the expander button in and re-gauging. Viola! Instant better concentricity – what isn’t there to like about that!

Like many things, in reloading, a little knowledge can be a dangerous thing².

There are many other, interrelated factors that can come into play here. However, here we are just going to focus on one little issue.

Measuring Concentricity and the neck turning conundrum

When we measure case concentricity, in the case of a neck, we are sometimes really measuring inconsistencies in the wall thickness³

I know, I know, many will say this is heresy (especially those already invested in the equipment) – but in short, if your necks are that bad, then that inconsistency likely carries on all the way down through the case – so you probably should just start with better cases and through the really bad outliers away!

Anyhow, the notion that removing the expander ball improves the concentricity of a neck may also be explained by this – when we resize brass – we are not removing any material. Can’t happen. What can happen, is the surface contacting the hard steel surface, the brass will conform in shape to it – any excess material will still have to come somewhere – and the simple explanation is that it goes to the other side of the case. That is, a resizing die pushes any excess (i.e. inconstancies) into the internal side of the neck, then the expander ball pushes it back out. Removing the expander die then simply leaves these inconsistencies on the inside of the neck.

The question

Now, the million dollar question – do these inconsistencies in the neck, affect the seating and function of the projectile more when on the outside, or the inside of the neck?

But wait you say – then why don’t we just remove any inconsistencies through neck turning! Well, kind of. Though that’s really a different article.

1. Neck Bushing Sizing Dies are a separate subject, but are relevant here ↩

2. though more knowledge can sometimes just reinforce incorrect ideas – refer confirmation bias ↩

3. again, sometimes attempted to be dealt with through neck turning – another rabbit hole, that isn’t actually recommended by several reloading company experts – check out our interview with Robin Sharpless for an example ↩