Case mouth doughnut

[ToddKS]

Last night I was in the process of prepping some brass for my mighty 6.5 Creedmost and I noticed there was a flare at the case mouth on some of the cases after I ran the mandrel through. The flare was big enough that the neck would not go into my case trimmer.

I ran the brass back through my neck bushing die and it went away. Ran the mandrel through and it came back. This made it clear that I was moving a ring of material back an forth. It was not something caused by one of the dies. I got out my neck turner and turned the necks on the affected brass. It clearly removed a ring from the outside of the top of the case mouth. After that I ran a couple test pieces back through the dies and had no issues with the flaring.

Anyone else ever experience something like this? I have been loading rifle for over 10 years and this is the first time I have every seen this. The only thing I can think of is that this was caused by peening during the cleaning process (I tumble with Southern Shine chip media).

Brass is Lapua, Redding body and neck dies, 21st Century mandrel and die body.

 

[piie]

Are you not perhaps ramming the brass into the end of the mandrel?

 

[dmoran]

A flare at the case mouth leads me to wonder if your cases were long for the chamber, hence getting flared inward when last fired.
May want to check your case to chamber length relationship.

 

[ToddKS]

Are you not perhaps ramming the brass into the end of the mandrel?

That was my first thought but that was not the case.

My die setting for the mandrel had not changed and I confirmed that I am well short of bottoming out. I even turned the die body out another turn just to confirm that is wasn't a certain spot in the mandrel.

 

[6MMsteve]

I take a good magnifying glass and chk the top of the outside case nk..there is excess material there...so when I prep my case I cut a heavy chamfer on all my brass this has done away with the material on the nk...when I run my finger over the nks now it is smooth...all new brass has this but a heavy chamfer takes this away for me..my nks look round at the top but they are smooth...nk turning doesn't really remove it, as I have found out..also I think this causes nk tension variables at the mouth area...if this makes sense

 

[ToddKS]

A flare at the case mouth leads me to wonder if your cases were long for the chamber, hence getting flared when last fired.
May want to check your case to chamber length relationship.

Good thought, but if anything these cases were a little on the short side. This was the 5th loading for this brass. I had loaded rounds that I checked and they did not show any signs of this.

 

[ToddKS]

I take a good magnifying glass and chk the top of the outside case nk..there is excess material there...so when I prep my case I cut a heavy chamfer on all my brass this has done away with the material on the nk...when I run my finger over the nks now it is smooth...all new brass has this but a heavy chamfer takes this away for me..my nks look round at the top but they are smooth...nk turning doesn't really remove it, as I have found out..also I think this causes nk tension variables at the mouth area...if this makes sense

This makes very good sense. I put a solid chamfer on my cases but I have been doing so after the mandrel step. I bet if I chamfer after the neck is sized but before I run the mandrel that that problem will go away. I will be removing the material from the inside before it gets pushed to the outside.

Thank you for this suggestion.

 

[Uncle Ed]

Question, are you wet tumbling your cases, it sounds like case mouth peening.

I have to trim my cases every time I wet tumble to remover the thick case mouth and then deburr.

 

[rwj]

Do you own, or have access to, a wall thickness micrometer?

 

[billbonser]

This occurs due to tumbling your brass. The longer the case touches other surfaces, the more the case mouth will get distorted. A good friend of mine noticed this with some brass he accidentally tumbled too long, he was using cob I believe. I have noticed this problem with stainless steel pin thumbing, and have to turn off the "bump". Bottom line is closely watch the tumbling time.

 

[ToddKS]

Question, are you wet tumbling your cases, it sounds like case mouth peening.

I have to trim my cases every time I wet tumble to remover the thick case mouth and then deburr.

I am wet tumbling. I found these when I was trying to trim them. The doughnut was preventing the case from going into the trimmer.

I agree that it sounds like peening as well.

 

[ToddKS]

Do you own, or have access to, a wall thickness micrometer?

I do, but in this case the defect was significant enough that I didn't bother to measure it. Case necks are turned to approx. .0135 thickness. When I turned the necks only the tip of the case mouth showed material being removed.

 

[ToddKS]

This occurs due to tumbling your brass. The longer the case touches other surfaces, the more the case mouth will get distorted. A good friend of mine noticed this with some brass he accidentally tumbled too long, he was using cob I believe. I have noticed this problem with stainless steel pin thumbing, and have to turn off the "bump". Bottom line is closely watch the tumbling time.

That is what surprised me about this. I read about people wet tumbling for a couple hours. I only run mine for 20-30 minutes normally. I was surprised that I was having issues on such a short run time but I suppose the effect is cumulative.

 

[rogina]

That is what surprised me about this. I read about people wet tumbling for a couple hours. I only run mine for 20-30 minutes normally. I was surprised that I was having issues on such a short run time but I suppose the effect is cumulative.

I wet tumble with ss pins. If I don't stop at 1 hour, it will peen the mouth of the cases.

 

[243winxb]

A bushing sizing down more then .008" may leave a flare on the case mouth.
Sizing down in 2 steps seems to help.

Ran the mandrel through and it came back.

This seems different? Could brass memory be an issue?

 

[carlsbad]

I recognized this as a wet tumbling problem when i read your first post. I don't wet tumble rifle brass any more. Those who do it successfully use about 4x the amount of media recommend (20lb vs 5 lb) and they use the chips instead of pins.

Otherwise, you're lips get beaten up.

--Jerry

 

[rwj]

Why would tumbling cause a donut to form at the start of the case neck?

 

[carlsbad]

It just gets beat up. The frayed end may be .025 when neck thickness is .014. --Jerry

 

[rwj]

It just gets beat up. The frayed end may be .025 when neck thickness is .014. --Jerry

Then the neck had to get shorter, an attribute that can be measured...

 

[carlsbad]

not really measurable until you trim it. Beat up the end of a stick, it will get longer until you clean it up.