Copper build up at base of case transition step.

[Donkey]

Hi folks.

I am seeing some odd copper build ups at the base of the case transition step in my .308 Win chamber on my Palma rifle.

I am using moly projectiles and Lapua brass. I recently trimmed my cases as I thought I was running out of head space but it seems I was bumping into these copper build ups. They are in little clumps around the base of the case transition step.

Has anyone seen this before or have an idea on what might be causing it?

 

[jonbearman]

Re: Copper build up in free bore.

How hot are your loads?

 

[Donkey]

I am using a lot of slow powder. Low pressure signs but high velocities. The throat and lead are showing signs of fire cracking.

 

[22BRGUY]

I suspect throat diameter contact, and would suggest possibly these 5 things as to why:
- a carbon build up, that is restricting the throat
- throat diameter is on the small side
- the bullets being used band/bearing are on the large side
- excessive run-out in the loaded ammo, causing excessive contact with the throat
- a ruff throat caused by tooling or chips when chambered
I suggest to make a chamber cast, to see what you have for specs and surface finish.

Let us know what you come up with
Donovan

Great info for me to Donovan...Thanks for help

 

[Donkey]

Re: Brass build up at base of case transition step.

Thanks Donovan.

Things I can rule out/investigate further. See in line.

I have been having trouble closing the bolt and then case ejection issues with this brass before I trimmed and shoulder bumped them. Could this be related? Just wondering how the case brass build up got there in the first place.

I have changed the subject as I think these are clumps of brass not copper at the end of the throat.

I suspect throat diameter contact, and would suggest possibly these 5 things as to why:
- a carbon build up, that is restricting the throat - Likely
- throat diameter is on the small side - Throat is on the large side
- the bullets being used band/bearing are on the large side - Yes end of run Lapua Scenars with slightly flared flats but then the throat is on the large side.
- excessive run-out in the loaded ammo, causing excessive contact with the throat. Run out is 0 - 0.0005" for each round - all are checked before firing.
- a ruff throat caused by tooling or chips when chambered - Possible and I am using an ejector spring which may drag the neck of the case over the throat of the chamber. It looks pretty good through the bore scope though...
I suggest to make a chamber cast, to see what you have for specs and surface finish.

Let us know what you come up with
Donovan