Black necks on brass

[Super Scout]

Have several 308 winchester cases where the neck on the outside is black looks like burnt powder. These were a hunting load with 168 grain Sierra Game Kings using the maximum load from the Sierra manual. I normally use a 175 gr Sierra match and found this blackening quite odd as its never happened with any of my match loads. Any ideas?

 

[Obtunded]

Is the pattern all around the case neck or is it along one edge?

It sounds like incomplete obturation. When the round fires, the neck of the case expands against the chamber wall and generally seals (obturates) against the chamber wall. If the load is light, if the bullet is seated too long , if neck tension is incorrect, or if the bullets are a larger diameter than the barrel (I know of a fellow having this happening with some 284 Berger bullets) it can cause an excessive amount of gas to blow back along the mouth of the case and cause a scorching pattern.

Is your bolt showing more carbon than usual?


Here is some crap regarding obturation...

"...But the consensus appears to bear out that the correct pressure to fully obdurate a
bullet to the bore reads like this; “…the approximate chamber pressure needed to fully
obdurate a bullet is a factor of 1,422 times the Brinnel hardness (BHN) of the
projectile…”. For example, a bullet of 19BHN would require a chamber pressure of
27,018 pounds per square inch (psi) (note that this reading is in psi, NOT CUP, Copper
Units of Pressure, as are readings in most loading manuals representing chamber
pressures"

 

[colinjw]

I agree with the comments made by Obtunded. I have a Russian Makarov Pistol in 9x18 that has the usual " generous " chamber that the Russians like to use to get reliable feeding. All my loads, commercial & my reloads come out with the brass looking like it came out of a camp fire. Hot gas coming back & scorching the case. Safe loads so I am not concerned. But anything that does not look or feel normal should be questioned.

 

[bigbull]

It is normal to have some dark deposits on the necks as long as they go no further than the neck shoulder junction. Some solutions are to apply a light crimp, seat the bullet closser to the rifling or reduce your expander ball diameter to increase friction between the bullet and the neck, a faster burning powder also will work. This is common with thin necks and long bullet jumps where peak pressures happen too far down the barrel or when the necks are too work hardened and loose their spring preventing good obturation.
bigbull

 

[prosper]

Here is some crap regarding obturation...

"...But the consensus appears to bear out that the correct pressure to fully obdurate a
bullet to the bore reads like this; “…the approximate chamber pressure needed to fully
obdurate a bullet is a factor of 1,422 times the Brinnel hardness (BHN) of the
projectile…”. For example, a bullet of 19BHN would require a chamber pressure of
27,018 pounds per square inch (psi) (note that this reading is in psi, NOT CUP, Copper
Units of Pressure, as are readings in most loading manuals representing chamber
pressures"

This is to ensure that sufficient pressure is present to obturate a cast bullet, not the case neck. Cast bullets often perform poorly with reduced loads of slow powders because insufficient pressure is created to obturate the bullet in the barrel and prevent gas cutting/blowby.

Carbon on the case necks is normal. Carbon on the case shoulder and body is a sign of insufficient pressure.

Uneven carbon on the neck could be a sign of a burr or nick inside the chamber, or dented brass (have you been lubing the necks/shoulders?)