30br Case Neck Questions

[Olde Man]

The necks on my fired cases have a sine wave carbon pattern on them. The necks have about .0015 clearance to the chamber. What causes this?
The other question is should I brush the inside of my case necks and why? I don't tumble or clean my cases other than a couple of twist with steel wool to remove the sine wave carbon pattern on the outside before I size them.

 

[Toby Bradshaw]

What causes this?

From Tony Boyer's "Book of Rifle Accuracy" (p. 149):

"A wavy pattern indicates that, as the bolt lugs settle on the receiver lugs, the brass has contracted enough for some gas to escape. Brass run out [sic] and lug engagement determine how even the wavy pattern is. As a side note, if you have very well matched lugs and straight brass on a 2-lug action, you will see two dips in the pattern. If it is a 3-lug you will see three dips (again if the lugs are a perfect fit)."

 

[Toby Bradshaw]

The other question is should I brush the inside of my case necks and why? I don't tumble or clean my cases other than a couple of twist with steel wool to remove the sine wave carbon pattern on the outside before I size them.

I brush the inside of case necks with a worn nylon brush. Don't know if it really matters.

I wouldn't use steel wool on the outside of case necks, though -- too abrasive. I use Nevr-Dull.

 

[Geronimo Jim]

Sinclair has the neck brush you need to brush the necks. Also, they sell crazy cloth to clean
the outside of the necks. I do both steps after shooting rounds. I learned this from a
record setting benchrest shooter.