Excessively narrow neck. What could cause this?

[Pond, James Pond]

I've been loading my next OCW set with Lapua 170gr Lockbases with N140 in my Norma .308 cases.

Cases were kept after shooting factory Norma ammo.

I shoot a batch of cartridges. I decap in batches, so not individually. Resizing occurs at the same time, of course. I have used a collet neck sizer, but have recently opted for the full length die.

I cannot say which method applied to this case. I have not yet checked all my other cleaned, trimmed, sized and primed Norma cases.

Basically, seating one of the Lockbase FMJBT bullets, I vaguely noticed that I could see the edge of the boat-tail sitting above the mouth of the case as it went up into the die. The bullet was much stiffer to seat than others and the resulting cartridge as a tapered "waist-line" hugging the boat-tail where the neck meets the shoulder!

I have measured the cartridge with calipers. The bullet is indeed .308, the neck around the bullet is .333", but the tapered section which has the boat-tail passing through it measures .320". A regular resized case ready for powder and bullet measures .331".

I will have to pull this bullet as it will be useless as part of an OCW test and potentially a cause for a pressure spike in a nearing-max charge.

What could cause this to happen to one case yet not to any other since I've been reloading?

[Barnacle Brad]

Measure the expander ball? At least to eliminate one possibility.

[Catfish]

What you are describing often happens with dies that have an expander ball that is on the small size. If you are loading max. load you might want to drop you charge and work up again to max, but it should not you load and the pressure might be up a little but not a lot, but if your on the edge a little can be to much.

[Bart B.]

Is the back half of the lock base bullet body bearing surface smaller in diameter than the front half? That would explain the neck diameter differences.

[Catfish]

Mic your bullet and check, but I am guessing what you are seeing is behind the bearing surface of the bullet and over the boat tail.

[Pond, James Pond]

I've checked my stock of primed cases and I've found a further two cases where the internal diameter of the mouth is about .283", as opposed to the .303 in properly sized cases.

I still don't see how this could have happened, particularly as I don't ever recall resizing only three cases: the sum total of the affected cases.

The same dies will have been used for all my cases.
So if it is an expander issue, then they all should have fallen foul, surely...

Is the back half of the lock base bullet body bearing surface smaller in diameter than the front half? That would explain the neck diameter differences.

They are boat-tails so they do taper down, but the mid section seems uniform in diameter. However, there still remains the fact that none of the other cases have suffered from this.

If you are loading max. load you might want to drop you charge and work up again to max, but it should not you load and the pressure might be up a little but not a lot, but if your on the edge a little can be to much.

Any of the cases showing this will not get shot. They'll be pulled, resized and reloaded at a later date. At the very least, being an OCW test, I don't need additional variables such as extremes of neck tension thrown into the mix! Different charge weights and my temperamental marksmanship are already two sizeable variables, and only one of them is useful to me!!

[243winxb]

Defective brass..

[Jimro]

Need to anneal. The brass has developed excessive spring back and will likely split soon if you don't.

Jimro

[Pond, James Pond]

Need to anneal.

And if i don't know how to do that, can I assume it is a case of binning those cases?

[higgite]

Doing the math from post #1, the average neck thickness of your brass is about .0125", which sounds pretty thin to me, but that's another story.

In post #6, you found 2 cases with ID of .283". Adding neck thickness to that gives an OD of .308". I don't see how a standard, non-bushing FLS die could size the neck down that far, even with the expander removed. My Forster die, for instance, sized necks down to .326" OD before I had it honed larger. That's a long way from your .308" neck OD.

You also mentioned having a Lee collet neck sizing die. My Lee collet neck sizer mandrel is .3055" OD, so using that die couldn't account for the small neck diameter either unless the die had the wrong mandrel in it. What am I missing here?

[Pond, James Pond]

What am I missing here?

Exactly what I've been asking myself.

I can have another look at the cases in question at home to give more/confirm the details.

[Jimro]

Pond,

Do you have a gas range in your home? Or a propane plumbers torch?

You don't have to heat the brass until it is red, as all you really want is stress relief and not to go "dead soft." On freshly polished brass it is easy to see the "blue color change" that lets you know you've reached an appropriate temp.

http://www.6mmbr.com/annealing.html

This guy drops the brass into water, although he really doesn't need to:

https://www.youtube.com/watch?v=MVeRDAsrCfM

Hope this helps.
Jimro

[Pond, James Pond]

Thanks for the links.

I will investigate further first, but if it starts happening to more brass, then annealing may be the answer.

[Pond, James Pond]

Well, it would appear I have found the problem.

A while back I snapped the decapping pin from my collet die. I had removed the expander to take measurements for the supplier. I think I must have resized the neck on those three bullets, perhaps after pulling bullets from cartridges that were too hot during my 150gr FMJ OCW test, but done so without the expander in situ, having forgotten to replace, given that I almost exclusively use the full-length die now.

That is my hypothesis and I will use the collet die, with expander in place to see if those cases return to normal .308 dimensions.

Honestly.... .
After all the times that a problem has been traced back to operator error, you'd have thought I'd have started there!!

[Unclenick]

James,

It sounds possible to me that excessive drag between the expander and neck actually pulled the neck out thin. That can pull the shoulder forward, too, if the brass is soft enough. If this happened, the two symptoms you would see are the case length being longer before trimming than the other cases were. The second would be that the mouth would be wider than the middle of the neck, as the expander would not be able to hold onto the last little bit as firmly as it did the middle.

To salvage the cases, run them through the collet die again. If expander stretching proves to be the culprit, clean the necks internally by turning a bore brush in them before sizing them again.

Originally Posted by Pond, James Pond

I decap… Resizing occurs at the same time, of course.

Actually, you can buy a universal decapping die from Lee. Also a hand decapper is made. So you can decap independently of sizing. Some do this to clean primer pockets. Others do it when they are cleaning cases by one of the liquid methods, as liquid can get into the flash hole and get caught under the spent primer and not dry fully by the time you next start to resize.

If you want to try annealing just a few cases, a slow but reliable method described by Fred Barker twenty years ago is to hold the case head between thumb and index finger and roll it back and forth while moving the neck and shoulder through a candle flame. When the head begins to become too hot to hold onto, slap the case onto a damp towel to cool it enough so you can grasp it again and rub the rest soot off on the towel. The candle is unable to overheat the brass, and the slap down, as opposed to submersion, doesn't get the inside wet, so you can dry the outside and not have to wait around for the inside to dry before loading and shooting it again.

[Pond, James Pond]

Thanks, Unclenick for the additional info, and indeed to everyone for their input.

I think this is the mostly likely explanation and I will put it to the test ASAP. Once I have, I can report the results.

On top of that I have several annealing references that I think, one day, I will have to try my hand at: it seems a good way to get the most from one's brass.