Case mouth shrinking?

[InkedIan]

I'm having an odd occurrence with my fired brass. The necks are tight right at the opening of the case mouth. Measured .003-.004" smaller at the opening of the neck than the rest of the neck. I have the reamer and the neck is straight .257" and the cases are short enough that they aren't being pushed into the freebore area. Has anyone ever noticed that before? Could it be caused by back pressure?

 

[dolek]

Are you using light loads ?

 

[InkedIan]

Are you using light loads ?

Mild load but not super light. 40.5gr h4831sc, 95gr smk in a 22 Creedmoor

 

[whatsupdoc]

Maybe carbon ring.

 

[InkedIan]

Maybe carbon ring.

Nope, borescoped it and it has less than 100rds on the barrel

 

[whatsupdoc]

What brass are you using?

 

[whatsupdoc]

Oh yea and the reamer at the neck should be around .290

 

[243winxb]

Could be how the brass was sized down from 6 mm ?
Or is it factory new brass.

 

[InkedIan]

It's Lapua 6 Creedmoor brass full length sized to 22 Creedmoor and the necks are turned. The neck of a loaded round measures .252 and the chamber neck is .257.

 

[searcher]

Common occurrence, especially as the brass hardens with use. When you size brass - you will find that it is almost always ends up smaller at the mouth - as the portion of the neck closest to the mouth is "unsupported" as compared to the portion of the neck which adjoins the shoulder area after sizing. the more you size the neck down - the more you will se this as well. When you fire, the entire neck opens up - then springs back to loosely approximate the slope it was sized at. You can minimize this by keeping your necks annealed, not oversize, not use a tight expander ball - and good neck lube. That said - a little bit of that condition exists in the best of loads.

 

[243winxb]

I dont have an answer why.
I sized a fired 308 down in a 243 bushing die, with no bushing installed. The mouth flared out like a funnel.

I would guess to much sizing in 1 step?

 

[whatsupdoc]

My mistake I misread, thought it was a 25 creedmoor.

 

[whatsupdoc]

So if I am understanding correctly after you resize and turn the neck the neck dimensions are correct but after you fire the case the neck issue appears?


I would take a couple of pieces of virgin brass and go through the entire process carefully measuring after each step to see when the issue occurs. First firing, second, whatever.

 

[David Christian]

Sounds like you simply need to skim turn them. I find the same thing after a few firings.

 

[Uncle Ed]

The polarity of your cases are reversed and your donuts are forming at the case mouth.

Reverse the polarity and the brass will flow in the other direction.

Sorry I've been locked up inside the house too long and have reread too many F. Guffey postings.

 

[bozo699]

The polarity of your cases are reversed and your donuts are forming at the case mouth.

Reverse the polarity and the brass will flow in the other direction.

Sorry I've been locked up inside the house too long and have reread too many F. Guffey postings.

That’s just to funny!....
Wayne

 

[dolek]

The polarity of your cases are reversed and your donuts are forming at the case mouth.

Reverse the polarity and the brass will flow in the other direction.

Sorry I've been locked up inside the house too long and have reread too many F. Guffey postings.

Aspiring to be Guffey, now I get it...

 

[bugman]

your question was answered by searcher. prove it to yourself with a measurement on the first firing after an anneal. Another pre and post measurement to factor in would be brass thickness at case mouth compared to thickness 2/3down the neck