Sooty case neck

[Thumper580]

Probably been asked already but I couldn't find anything.
I reload for 308 using imr4064, 4895 and Varget... 155's, 168's and 175's.
With various load combinations the case necks are sooty after firing and other loads aren't (same brass).
Is this simply a function of pressure where some are sealing better upon firing and are not sooty?

 

[Emerson0311]

Probably been asked already but I couldn't find anything.
I reload for 308 using imr4064, 4895 and Varget... 155's, 168's and 175's.
With various load combinations the case necks are sooty after firing and other loads aren't (same brass).
Is this simply a function of pressure where some are sealing better upon firing and are not sooty?

Pressure too low for the sooty loads to seal the neck in the chamber. Been there. Up the pressure.

 

[Marine52]

Probably been asked already but I couldn't find anything.
I reload for 308 using imr4064, 4895 and Varget... 155's, 168's and 175's.
With various load combinations the case necks are sooty after firing and other loads aren't (same brass).
Is this simply a function of pressure where some are sealing better upon firing and are not sooty?

Carbon Soot on the neck does not necessarily mean you have a problem. There is good soot (sine pattern) and bad soot (black smudge line further down the neck all the way around and even as far down as the shoulder or a clear horizontal carbon soot line just below case mouth)

Wavy marks (many experienced comp reloaders refer to this as a carbon sine wave pattern) are normal and show that you have good clearance between the case neck and chamber. The sine wave shows that some of the gases escape into the chamber before the neck actually expands completely, sealing the chamber. This proves that you are getting a good bullet release.

Too little clearance between the case neck and chamber will cause the soot carbon ring around the case mouth close to the end of the neck like figure 1 in picture below. Too much clearance between the case neck and chamber will show the black smudge line further down the neck all the way around and even as far down as the shoulder.

Tony Boyer describes this in his book, ”The Book of Rifle Accuracy,” on page 149:

Here’s an example of a sine wave pattern of carbon soot, indicating good clearance between the case neck and chamber: