Growth in Case Neck Thickness - what causes it ?

[Timo Turl]

I’m trying to understand what are the key drivers of growth in case neck thickness and what steps can be taken to minimise it ?

I notice that in a couple of my rifles case neck thickness on Lapua brass is only slightly thicker than new brass even after 6-8 firings. However, on a 6.5CM the case neck got much thicker and at a much faster rate such that I couldn’t chamber a loaded round without neck turning. Never had that happen before.

What’s causing that expansion in the case neck and are there steps that can be taken to minimise the pace of growth so as to reduce the frequency of having to turn necks ? Just wondering if it’s the chamber, excess pressure, reloading technique or if some dies are more ‘guilty’ than others etc ? Thanks !!!

 

[Supersubes]

What are you measuring with, and where exactly on the neck are you measuring?

 

[Timo Turl]

I’m using a K&M Mitutoyo Wall Thickness Micrometer and I’m measuring along the case wall neck. Measurements are reasonably consistent along that length.

 

[straightshooter1]

Are you FL sizing and using an expander ball in the die?

Typically, it's not the firing of the round that'll do much of this as the case expands to fill the chamber and the pressure is equal over the entire interior of the case. Resizing techniques are what causes changes in all the dimensions of a case as brass is relatively soft and kinda act like clay when using presses to reform them.

PS: Also, what is your brass cleaning method? Carbon build up also adds to the neck thickness over time if not thoroughly cleaned after each firing.

 

[Timo Turl]

Yes a FL die with an expander ball and I use the same type of die in all my rifles yet the case neck thickness growth in one is significantly more than in the others.

 

[Supersubes]

I can’t say that I’ve ever seen necks uniformly thicken across their length. The material would have to come from somewhere. The closest I’ve seen is a donut forming over many successive firing/sizing/trimming operations, where the thicker shoulder material has moved forward into the base of the neck.

Is your chamber tight necked? There’s about .005-.007 of clearance in a saami creedmoor chamber(depending on brass). Seems impossible that necks could thicken so much as to impede chambering. Just how much thickening are you measuring?

 

[Timo Turl]

From 0.014 growing to 0.0165 x 2 + .264 = 0.297 vs my 0.295 neck but even 0.297 is pretty tight for a non-tight necked chamber.

Even if we take just the mouth of the case, the neck is 0.016 to 0.0165 so we don’t need to even get to the topic of a donut. When I turn the necks, I get a nice clean cut around virtually all the neck ie no cold / hot spots when I take it back to 0.014

I am just trying to understand what causes that growth. I’ve never seen a case grow so much.

 

[straightshooter1]

From 0.014 growing to 0.0165 x 2 + .264 = 0.297 vs my 0.295 neck but even 0.297 is pretty tight for a non-tight necked chamber.

Even if we take just the mouth of the case, the neck is 0.016 to 0.0165 so we don’t need to even get to the topic of a donut. When I turn the necks, I get a nice clean cut around virtually all the neck ie no cold / hot spots when I take it back to 0.014

I am just trying to understand what causes that growth. I’ve never seen a case grow so much.

It appears that it's very likely the expander ball. It may not be the right diameter for your trimmed necks. When the ball is pulled up through the neck after the neck has been squished down, it draws brass from the shoulder area and puts in into the neck. This is also one of the reasons why the brass length his longer and will need trimming to maintain the length you had trimmed it to beforehand. To get the same thickness that you've turned the necks to, you'd need to have a expander ball that's slightly larger so that it'll make up that difference that you've cut away. And doing this, that extra brass that made up the thicker neck will now make the neck longer for a longer over all length of the case.

What a lot of reloaders do is they don't use an expander ball at all and will resize the case without it and then run a proper sized mandrel through the neck to get the neck tension desired. Another reason for this method is for achieving better concentricity.

 

[47guy]

It appears that it's very likely the expander ball. It may not be the right diameter for your trimmed necks. When the ball is pulled up through the neck after the neck has been squished down, it draws brass from the shoulder area and puts in into the neck. This is also one of the reasons why the brass length his longer and will need trimming to maintain the length you had trimmed it to beforehand. To get the same thickness that you've turned the necks to, you'd need to have a expander ball that's slightly larger so that it'll make up that difference that you've cut away. And doing this, that extra brass that made up the thicker neck will now make the neck longer for a longer over all length of the case.

What a lot of reloaders do is they don't use an expander ball at all and will resize the case without it and then run a proper sized mandrel through the neck to get the neck tension desired. Another reason for this method is for achieving better concentricity.

this makes sense the way its explained "and im not saying its not possible" but ive been using the expander ball in my dies for at least 14yrs and in several different calibers and have never seen this happen.

i also bought a set of 21st century mandrels and redding dry lube...now granted i did not spend much time with them and i will get back to them in the next couple of weeks but i was less than impressed with the initial results.

 

[straightshooter1]

this makes sense the way its explained "and im not saying its not possible" but ive been using the expander ball in my dies for at least 14yrs and in several different calibers and have never seen this happen.

i also bought a set of 21st century mandrels and redding dry lube...now granted i did not spend much time with them and i will get back to them in the next couple of weeks but i was less than impressed with the initial results.

It's a really usual thing to see .0025 of growth in neck thickness, even over a number of firings. I've used Federal brass, which is just about the softest cases on the market, and haven't gotten much growth. So, I do wonder to about the dimensions of his chamber, whether there may be a contributing factor there.

 

[spife7980]

I’ve heard of necks thinning over several sizing and firing cycles but not ever thickening aside from shoulder material migrating forward.

 

[Foul Mike]

I had a 98K 8mm Mauser years ago that did this. It was the barrel and chamber from the factory, a surplus rifle, and I was using RCBS dies.
I was really new to reloading, first rifle I reloaded for and I was poor as a church mouse so was using the most rudimentary of tools
It took quite a few firings for it to show up but it did.
I never did figure it out and at the time thought that was just how things go.
I haven't run into it much since then and my biggest problem with brass now is primer pockets.
Also shooting much nicer rifles, that may have something to do with it.
I think maybe a war time rifle with slightly loose chamber is what I had going on. FM
PS The only brass failures I had with that 8mm was an occasional split neck and I was reloading that brass waaay more times than I would now. I was Po.

 

[47guy]

It's a really usual thing to see .0025 of growth in neck thickness, even over a number of firings. I've used Federal brass, which is just about the softest cases on the market, and haven't gotten much growth. So, I do wonder to about the dimensions of his chamber, whether there may be a contributing factor there.

yeah i wonder about the chamber myself...i FL size with the expander ball and trim my brass every every time...i get just enough growth that my trimmer just cleans up the mouth but when i anneal it takes just a little brass off and like @spife7980 said my necks get thinner.

 

[Milo 2.5]

I’ve heard of necks thinning over several sizing and firing cycles but not ever thickening aside from shoulder material migrating forward.

Necks always thin on first firing, why a guy should never buy a bushing based off loaded rd with new brass. If brass is actually getting thicker, which I guess I have not experienced, I would point a finger at running expander balls in dies.

 

[Timo Turl]

Sorry, to be clear there is no expander ball. I use a Forster Bushing Bump die and I use the same in my .308 and .260 which doesn’t show growth.

I was thinking it could be one or more of the following:

1. The die. It did have to go back to Forster because it was too long / wouldn’t bump the shoulder at all. They took a little off the bottom they said to get it to bump the required 2 thou. Forster bump dies don’t size the body so would that matter ?
2. The chamber. It is a tight neck at .295 which leaves me with only 0.003 of clearance with new brass.
3. Brass didn’t grow too much in length, didn’t need to trim in 3-4 firings.

I’ve now FL resized (not bumped), trimmed, annealed, turned necks and it chambers like a champ. I’m also moving over to Redding S Type dies without the expander so will see if that makes a difference.

Weird. My dies/technique are exactly the same across the 3 calibers and the only thing that is different is the tighter neck on the 6.5CM and the die having that original issue.

 

[straightshooter1]

Sorry, to be clear there is no expander ball. I use a Forster Bushing Bump die and I use the same in my .308 and .260 which doesn’t show growth.

I was thinking it could be one or more of the following:

1. The die. It did have to go back to Forster because it was too long / wouldn’t bump the shoulder at all. They took a little off the bottom they said to get it to bump the required 2 thou. Forster bump dies don’t size the body so would that matter ?
2. The chamber. It is a tight neck at .295 which leaves me with only 0.003 of clearance with new brass.
3. Brass didn’t grow too much in length, didn’t need to trim in 3-4 firings.

I’ve now FL resized (not bumped), trimmed, annealed, turned necks and it chambers like a champ. I’m also moving over to Redding S Type dies without the expander so will see if that makes a difference.

Weird. My dies/technique are exactly the same across the 3 calibers and the only thing that is different is the tighter neck on the 6.5CM and the die having that original issue.

I also use a Forster Neck Bump Die and also use it with the expander ball removed. I've used it with and without the bushing to see which produces less runout. For the most part, I use it without the bushing too after I've neck sized cases with a collet die, the run a mandrel through it to get my desired neck tension. So . . . I've used this die in every configuration and haven't increases in neck thickness anywhere close to what you've experienced here.

Note, though this dies does not size the case body, it still sizes the neck and will increase the case overall length (depending on just how much the neck is being reformed). The more the neck is squeezed in the bushing that brass has to move somewhere, which is up away from the shoulder. . . and maybe (MAYBE) a little into the neck thickness as well.

Typically, I would think a tight necked chamber would be "a good thing" by reducing the amount of work being done on the case neck when resizing it. But, what about the rest of the chamber? I'm thinking that if the other parts of the chamber is relatively looser, like the shoulder area, than what you have in the neck, this might cause the brass to flow neckward more. Am not sure about this as I've never encountered this problem, so it might be worth checking out to see what clearances you have there.

Normally, I wouldn't expect the cases to grow much in length in a tight chamber. But, if the necks are growing in thickness as you've experienced, that additional brass is coming from somewhere and for some reason it's not being added as length.

 

[MikeMcCasland]

If it's the case mouth, it's wet tumbling peeing it, and causing it to mushroom ever so slightly. At least I've had it happen multiple times.

The actual neck, I haven't seen. Where the case neck meets the shoulder....well that's just a donut forming.