6BR - Residue on case neck

[Patruck825]

Never seen this with other calibers, I’ve got powder residue on my case necks and it looks kind of like a sine wave. Not on all of my cases, but the majority of them. I have heard people mention this before.

But, what’s the reason for the residue? Isn’t the neck supposed to seal during firing?

Reamer is .271 neck with blue box lapua brass

 

[rebel]

Don't worry about it, your good.

 

[Patruck825]

Don't worry about it, your good.

Well I assume that, it shoots great.

Curious is all, like to learn when I have the chance.

 

[geraldgee]

I've read but don't know it its true that the "sine waves' indicate where the action lugs are located. It would be easy enough to test that theory by indexing a couple of cases and see if the waves correlate in location to the lugs.
Good luck with it whatever you do.

Gerald

 

[Toby Bradshaw]

I’ve got powder residue on my case necks and it looks kind of like a sine wave.

But, what’s the reason for the residue? Isn’t the neck supposed to seal during firing?

The neck does seal the chamber while the bullet is in the barrel. The residue you're seeing comes from the blowby when the pressure drops enough to let the neck spring back. An even pattern on opposite sides of the neck (for a 2-lug bolt) indicates that the bolt lugs are seated evenly. There will be 3 "waves" with a 3-lug bolt.

 

[rebel]

Well I assume that, it shoots great.

Curious is all, like to learn when I have the chance.

I don't really have an explanation, I assumed yours was shooting well as my two do. Have the same wave pattern.

 

[Patruck825]

The neck does seal the chamber while the bullet is in the barrel. The residue you're seeing comes from the blowby when the pressure drops enough to let the neck spring back. An even pattern on opposite sides of the neck (for a 2-lug bolt) indicates that the bolt lugs are seated evenly. There will be 3 "waves" with a 3-lug bolt.

Very interesting, thanks Toby.

 

[Fred Bohl]

The neck does seal the chamber while the bullet is in the barrel. The residue you're seeing comes from the blowby when the pressure drops enough to let the neck spring back. An even pattern on opposite sides of the neck (for a 2-lug bolt) indicates that the bolt lugs are seated evenly. There will be 3 "waves" with a 3-lug bolt.

I've seen the wave pattern on many rounds but I do not understand how the neck would only spring back near the outer end. My observations indicate that there is some variation in how far down the neck the pattern appears and in the "peak to peak" magnitude even with the same case and load fired multiple times in the same chamber.