Mysterious brass shrinking… pls help!!!

[lcx99321]

I saw a video from Primal Rights where he talked about running tight neck tension and moving a large amount of material during case prep. The idea was that it helps smooth out imperfections in the brass. It made sense to me, so I gave it a shot. At first, it worked great—groups were solid.

But then I started noticing my brass was getting shorter with each firing—0.002” shorter every firing. Shoulder measurement hasn’t changed across the 3 firings. and it’s super consistent across the whole batch. By the third firing, case length was down to 1.8995”, and I started seeing some nasty carbon buildup around the throat area (I think that’s the right term—the little gap just in front of the case neck in the chamber).

Now my groups are going to crap. It still shoots tight for the most part, but I get a random flyer in almost every 3-shot group. The barrel has about 1,000 rounds through it, which seems a little early for the barrel to burn out.

So here’s what I’m trying to figure out:

1. Why is my brass shrinking? Could it be a donut forming? If I’m using an expander mandrel, it shouldn't be a problem to affect groups?
2. Are the flyers coming from the carbon buildup, or is the donut the real problem?

Here’s my setup:

• 25 Creedmoor, Lapua 6.5 CM brass
• H4350, 39.2 gr
• CCI 450 primers
• 2780 fps
• SD around 4–6, ES about 12–15
• Berger 135gr LRHT, seated 0.020” off the lands
• Anneal every firing
• Loading on a Dillon 550
• Full length sizing with a Whidden bushing die
• .276 bushing followed by a .254 expander mandrel

Thanks in advance for any input!

 

[TheCZKid]

I suppose the donut could be causing the necks to bump up against it, and push against the brass to stop it short, thus causing the neck to push back slightly.

I'd think cleaning out that donut would be a definite thing to do to eliminate that as a variable. I'm sure lots of guys here can share their best methods to be effective. I have mine, but I'm not sure it's the best.

 

[FeMan]

As you surmise, if the cases are really shrinking, the brass is going somewhere. In the first firing and perhaps the second, fire forming to the chamber will reduce overall case length. That could be the source of some of it. But by the third firing it should be static or growing with sizing as you are expecting.

Bushing dies promote donut formation. Pulling an expander button through should push all or most of that material to the outside of the case though. As long as your bullet base is above the donut, it shouldn't be a factor. Are you above the donut? If not, you will feel it when seating. If you do have a donut that is influencing seating, I would suggest skim turning your necks after you size and have pushed the material to the outside.

A carbon ring could definitely affect groups. If that isn't the problem now, it is coming. You need to get that out and up your cleaning to stay ahead of it.

At 1,000 rounds, your load could need a tweak. One characteristic of tune I see is that when a rifle is close to tune, it will develop a pattern of a centroid with a shot that orbits the cluster and appears as a "fyler". As it drifts farther out of tune, the whole group will scatter. I would suggest that you shoot five loads with one being your current, and for the the others add/subtract 0.003" and 0.006" from the CBTO measurements. I have attached a picture of a load development I am working on right now that illustrates that. The center is in tune and the on the left and right you can see a group with a "flyer" in orbit. IME these flyers are not random.

Hope that helps.

 

[Sandstorm]

Yep a good cleaning program will certainly help.

ME these flyers are not random.

Good explanation and ive seen it before as well.

 

[CF Junkie]

You might check to see if your full sizing die is actually setting back the shoulder by about 0.002.
I have found that when my brass started shrinking, I wasn't seating the shoulder back when I was resizing.

 

[243winxb]

I started noticing my brass was getting shorter (trim length) with each firing—0.002” shorter every firing. Shoulder measurement (head to datum) hasn’t changed across the 3 firings

expander mandrel

Are you lubing the inside of the necks. With what? The mandrel seems to be pushing the necks back/shorter?

Set the bushing to size for .002" neck tension Skip the mandrel. Add neck turning to the brass prep.

OR use a bushing that only allows the mandrel to expand .001"
STOP over working the case necks.

Better accuracy is achieved by sizing 1/2 of the necks. This centers the bullet in the chamber. After about 3 firing, the unsized neck has fully expanded to the chamber.
Donuts are no longer a problem.

 

[Will Henry]

Your brass is shrinking for a couple of reasons. One is that it well annealed, the other is that you are full length sizing. I'll add a third reason here; your load is relatively mild.
When you full length size you reduce the diameter of the case. Now, this normally lengthens the brass, but in some cases, if the brass is annealed and the case shape and die dimensions help control this, it may not. It would be educational to measure brass after sizing.
When the cartridge is fired, it expands to fit the chamber. Since it has become fatter, it must also become shorter. Now, normally the pressure is sufficient to keep the case tight against the bolt face or stretch the case until it contacts the face. Lacking sufficient pressure to do this, but sufficient pressure to grip the chamber wall, the base of the case will actually be drawn away from the boltface. In other cases, the cartridge may remain firmly seated against the face. As the case expands circumferentially, it shortens by pulling the neck back.
Careful measurement at each stage, before sizing, after sizing, and after firing, will help you to determine exactly what scenario is taking place here. WH

 

[222Jim]

I saw a video from Primal Rights where he talked about running tight neck tension and moving a large amount of material during case prep. The idea was that it helps smooth out imperfections in the brass. It made sense to me, so I gave it a shot. At first, it worked great—groups were solid.

But then I started noticing my brass was getting shorter with each firing—0.002” shorter every firing. Shoulder measurement hasn’t changed across the 3 firings. and it’s super consistent across the whole batch. By the third firing, case length was down to 1.8995”, and I started seeing some nasty carbon buildup around the throat area (I think that’s the right term—the little gap just in front of the case neck in the chamber).

Now my groups are going to crap. It still shoots tight for the most part, but I get a random flyer in almost every 3-shot group. The barrel has about 1,000 rounds through it, which seems a little early for the barrel to burn out.

So here’s what I’m trying to figure out:

1. Why is my brass shrinking? Could it be a donut forming? If I’m using an expander mandrel, it shouldn't be a problem to affect groups?
2. Are the flyers coming from the carbon buildup, or is the donut the real problem?

Here’s my setup:

• 25 Creedmoor, Lapua 6.5 CM brass
• H4350, 39.2 gr
• CCI 450 primers
• 2780 fps
• SD around 4–6, ES about 12–15
• Berger 135gr LRHT, seated 0.020” off the lands
• Anneal every firing
• Loading on a Dillon 550
• Full length sizing with a Whidden bushing die
• .276 bushing followed by a .254 expander mandrel

Thanks in advance for any input!

Each time you fire the cartridge, the case expands to fit the chamber. That increases the diameter of the case, albeit slightly. That brass has to come from somewhere, and it comes at the expense of a shorter case. Eventually you'll have to resize those cases when the "spring-back" just isn't enough to overcome that expansion.

Run the cases through a full body sizing die and you'll see you get longer cases.

FYI, because of this I never trim cases until I've fired them 3 or 4 times in my rifle, assuming they are all below the case maximum.

 

[Jeff Porter]

Clean up that carbon ring.

 

[J_idaho]

If I were you I would get a bigger bushing after you turn your necks another .001” to clean up the mess made by bushing. And for the love of Pete clean that carbon ring. Shrinking has to be that monster donut you have. Most of my donuts are not that visible. Clearly your tight bushing die is moving allot of material backwards. Maybe I’m wrong though. I have seen people use bushings locked down tight cause all kinds of neck problems but yours looks like you have it set really loose maybe to loose.

 

[lcx99321]

Are you lubing the inside of the necks. With what? The mandrel seems to be pushing the necks back/shorter?

I lube the inside of the neck with graphite dry lube, and imperial sizing wax for the outside

 

[lcx99321]

I have seen people use bushings locked down tight cause all kinds of neck problems but yours looks like you have it set really loose maybe to o loose.

That's one of my concern too. I see people say sizing 70% of the neck can avoid donut forming. But it looks like I'm creating a giant donut with the tight bushing.

 

[lcx99321]

As you surmise, if the cases are really shrinking, the brass is going somewhere. In the first firing and perhaps the second, fire forming to the chamber will reduce overall case length. That could be the source of some of it. But by the third firing it should be static or growing with sizing as you are expecting.

Bushing dies promote donut formation. Pulling an expander button through should push all or most of that material to the outside of the case though. As long as your bullet base is above the donut, it shouldn't be a factor. Are you above the donut? If not, you will feel it when seating. If you do have a donut that is influencing seating, I would suggest skim turning your necks after you size and have pushed the material to the outside.

A carbon ring could definitely affect groups. If that isn't the problem now, it is coming. You need to get that out and up your cleaning to stay ahead of it.

At 1,000 rounds, your load could need a tweak. One characteristic of tune I see is that when a rifle is close to tune, it will develop a pattern of a centroid with a sot that orbits the cluster and appears as a "fyler". As it drifts farther out, the whole group will scatter. I would suggest that you shoot five loads with one being your current, and for the the others add/subtract 0.003" and 0.006" from the CBTO measurements. I have attached a picture of a load development I am working on right now that illustrates that. The center is in tune and the on the left and right you can see a group with a "flyer". IME these flyers are not random.

Hope that helps.

That's great info! If you don't mind me asking, how do you check if the bullet is seating above the donut? Do you seat a bullet, mark the bearing surface and pull it out to compare?

 

[J_idaho]

That's one of my concern too. I see people say sizing 70% of the neck can avoid donut forming. But it looks like I'm creating a giant donut with the tight bushing.

Maybe with really light neck sizing. You always have to consider the source of your info. A competitive bench shooter is going to assume you understand that they are using a customer reamer designed around a particular bullet with the full intent on seating in front of the donut and only single feed mag. But for a normal mag fed rifle that may not work.