Pros and Cons of expander balls

[ColinR]

I am re-visiting some of my reloading procedures and an area I am uncertain about is the use of expander balls when resizing.

I use RCBS dies with removeable bushings and have a selection for each calibre covering 10 or 12 thou.

Given that case neck thickness can vary on a single case by +/- 0.005 or more it is clear that using a bushing to resize will accurately reform the outside of the neck to a perfect diameter, but due to the variations in brass thickness around the neck the inside will vary by the deviations in thickness and will not be symmetrical, causing uneven tension around the bullet (or does the bullet reform the neck and even out the tension during seating?)

Resizing with a bushing and expander ball will to my mind have the following effect:

The bushing will reform the outside of the neck to a given size and as the expander ball is withdrawn it will reform the inside of the neck (the important part!) to an exact symmetrical diameter with any fluctuations in brass thickness then going to the outside.

Obviously using variable bushings it is possible to accurately adjust tension at the neck (theoretically) but with the downside that all variations in brass thickness are pushed to the inside - in practice this may not matter, I am not sure, hence this question. However, it would seem that using inside expander balls of different sizes could result in a more consistent neck tension by cancelling out variations that exist around the necks of most cartridges.

[The Gun Pimp]

Ditch the expander-ball and neck-up with a separate neck-mandrel - available from Sinclair with holder for reasonable cost.

[rox]

Given that case neck thickness can vary on a single case by +/- 0.005 or more

Agreed - the Sinclair expander is great for making straight ammo with consistent neck tension. But do you really have brass with +/- 0.005" thickness? That's a total range of 0.010"; I'd be worried if the total spread was as much as 0.005", and even the best neck expander will induce runout when there is significant thickness variation, since uneven thickness leads to uneven expansion around the circumference. It sounds like you may need to either neck-turn or get better brass if you really have that much variation. With Lapua, RWS and MEN brass I have avoided neck-turning while maintaining good runout with careful use of the Sinclair expander, especially the carbide expander (which is intended for neck turning and slightly smaller in size than the stainless steel expanding mandrel, but the lower friction of carbide may help - especially if you don't deliberately and carefully lube the inside of each neck).

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[ColinR]

Yes, 0.005 is probably at the cheap end of the scale (I usually use Lapua, but had some free GGG cases some of which were quite irregular and measured with an ordinary vernier rather than the ball type which initself induces erroneous measurement, but still indicative of variation. Thanks for pointing to The Sinclair expander, not an item I know but will research.

[ColinR]

I just checked out the Sinclair mandrel system. I can see how it is good for preparing new unfired cases that might have more neck tension than required, but presumably it will add another process to sizing fired cases as the necks will need to be sized down first. What is the procedure here i.e. what size neck bushing relative to neck dimensions of brass to get best benefit from the mandrel?

[ColinR]

I am going to get the Sinclair expander with mandrel and try it with .308. There are two different mandrels, a s/steel one 0.001" (.307) undersize and the carbide version 0.002" (.306) undersize. rox seems to favour the carbide version and I can see the benefits of carbide/lube, but is this OK just for forming if I have no intention of neck turning. What sort of spring back to final dimension could I expect. I have to say I prefer the neck to be tighter, but with spring back will the neck be too tight - I'd expect spring back to be about 0.001" giving neck tension of 0.003" with the carbide mandrel. Is this a correct assumption?

And anyone know the best place to buy this kit?

[rox]

I have to say I prefer the neck to be tighter, but with spring back will the neck be too tight - I'd expect spring back to be about 0.001" giving neck tension of 0.003" with the carbide mandrel. Is this a correct assumption?

I had the same concerns and went for the carbide mandrel despite it being slightly below optimum size. I was very happy with the resulting runout as well as the performance on target. There are some third party mandrel-makers, but I didn't find a 0.307 carbide one. One day I may try and source one or have one made, but the Sinclair one is working so I'm in no rush. I'll try and remember to take some measurements to get a definitive answer on the springback etc, but I don't expect to be prepping brass for a little while.

And anyone know the best place to buy this kit?

I buy direct and ship to my bundlebox, usually batched together with some other orders.

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[ColinR]

Thanks rox that's good enough reason to go with the carbide mandrel. I'll order direct from Sinclair and have it shipped to my niece in US for onward despatch. I'll probably order both mandrels so I have the choice.

[Maggot]

Call me old mr thicky....

I have seen folk using mandrels, but have tended to just use a bush without an expander ball.

Why have different sized bushes if you then shove a standard single sized mandrel down the inside of the case?

Moreover, how do you then partially re-size the necks? I only do 75-80% of the neck length.

[ColinR]

Hi Maggot Its a difficult one; bushes will give an exact profile to the outside of the case, while a mandrel/ball will give a more exact profile to the inside of the case - the latter probably being the most important. You are right that bushes give a lot more options in sizing the neck, but only take account of differences in neck thickness, where the mandrel is directly related to the bullet dimension i.e. 0.001" or 0.002" smaller and I can see it provides more of a constant in neck tension than using different bushings to accommodate different dimensions of brass at the neck - particularly with different makes of brass. A mandrel will ensure the same neck tension whatever the make of brass or thickness at the neck.

Its always made more sense to me to size the inside of the case neck rather than the outside, although I have mostly used the bushing method. I now intend changing to inside sizing with mandrels and give it a go. I have to say I am convinced, but time will tell. I neck size the full length so using a mandrel will not change this. It'll be interesting to see what effects it has on the life of the brass. Presumably if you only resize the neck with a bushing to 80% the mandrel will only open up that much, because the un-sized portion will be bigger than the mandrel.