What are the causes of this?

[nicky]

     After shooting a cartridge, the case has burnt powder down from the neck. Other than a low charge or light load not making the case seal in the chamber what may case this?

[vwpieces]

You are probably right, low charge.

What is the cartridge & reload data?

[1SOW]

Yes,  a light charge can fail to make a seal,  but ALL charges will blow "some" powder residue 'back' as can be seen in slo-mo video.
Dirty powders blown back will amplify the resulting residue.
A tight seal limits the amount of residue from any given powder.
JMO

[Wobbly]

Too low of pressure. The inside of the case is probably also black. If both of those are true, then you can add a little more powder to up the pressure.

 

[nicky]

Thanks guys. That was my thought. Just wanted to be sure.

[Wobbly]

Thanks guys. That was my thought. Just wanted to be sure.

Considering the black colored case inside and out scenario....

If your accuracy is outstanding and your chrono tells you the bullet speed is what you want, then this is when (if times were better and powder more plentiful) you might want to try a slightly faster powder. The bullet speed tells you the pressure is about right, so what might be called for is a powder that will reach the same pressure with slightly more efficient combustion. Obviously, this is easier done within "families" of powders (say substituting N330 in place of N340, or No.2 for No.5) but it's a path that can be followed.

Or you can take the other route and add more of the existing powder. That will definitely get the powder you have into a more efficient burn range AND seal the brass against the chamber, but it will also raise the chamber pressure which in turn will raise the bullet speed. However, increased bullet speed may introduce its own set of issues: increased barrel leading and/or lower bullet accuracy. The drawback with the "add more 'til you get there" approach is that it may raise the chamber pressure too high and go over the pressure limit. Typically though, slow powders are much too voluminous and your calculations tell you to put something like 16.4gr into a 9mm case and it just ain't possible.

At least to me, this is how "fast powder" / "slow powders" play.

 

[painter]

Occasionally, I'll get a case that shows some soot on the outside even with my favorite VV N320 load w/ plated bullets.

I don't worry about it.

[M1A4ME]

Rifle brass is supposed to get harder the more times it is fired and resized.

Does pistol brass do the same thing? Or is the pressure/heat generated with pistol ammo/brass vs. rifle ammo/brass make it a non-issue?

If it does get harder then after a certain number of loadings would it be less likely to expand as much/quickly to seal the chamber against the burning powder/pressure?

I've noticed this way more with .45 acp than with 9MM or .40 S&W/.357 SIG.  But I still see it with rifle cases, sometimes.

[Lemming]

Brass work-hardens, whether in rifle or pistol cases. That means the more you use and resize the case the harder and more brittle they become. Eventually they tear instead of sealing against the chamber wall properly.

It is possible to anneal rifle case walls by heat treatment,  which makes them soft again. But the base of the case can't be annealed because that needs to be strong. Because pistol cases are so short I don't think it's possible to safely anneal the mouth while leaving the base strong and hard.

[Wobbly]

Brass work-hardens, whether in rifle or pistol cases. That means the more you use and resize the case the harder and more brittle they become. Eventually they tear instead of sealing against the chamber wall properly.

It is possible to anneal rifle case walls by heat treatment,  which makes them soft again. But the base of the case can't be annealed because that needs to be strong. Because pistol cases are so short I don't think it's possible to safely anneal the mouth while leaving the base strong and hard.

Well said.

If you could anneal the case mouth without affecting the base, it would probably be too time consuming. Such as standing the cases in a certain depth of water and carefully waving a torch around. The necessary payback and viability on cases such as 357, which are expensive and tall, would be much better than on the short, cheap 9mm.

 

[Wobbly]

Maybe I had too much coffee this PM, but I felt compelled to make another cartoon. I'm not fully happy with the results because while it demonstrates somethings well, others it does not. (I suspect that the real relationship is more 3 dimensional and therefore has no way of being faithfully represented.) This chart is purely a work of fiction. Even with this preface, some will probably see fit to poke holes in it. Have at it.

What I wanted to help Nicky understand was how the different powder speeds ("burn rates") worked together. Now everyone understands that powder "burn rate" is simply a quick way to relate powders. It is deeply flawed and inaccurate at best, but it's what we got and everyone uses it, even with the limitations.



What we can visualize is that Nicky might be using Powder D, and his resulting sooty burn and failure to seal might be caused by being near the starting load, which while "safe" and "workable" is far from "optimal". Even though the powders are not evenly spaced, you can usually slip left to one of the next faster powders (Powder C or Powder B) and retain the same bullet speed. This will allow the combustion to occur in the more optimal mid-range, thus eliminating the soot altogether by burning the powder far more efficiently.

We see also that bullet speed is closely tied to chamber pressure, but is not 1:1 with pressure in all cases. I am not a balistician, but assume that the methods used to make powder burn faster also make the shape of its pressure curve change too.

Nicky could also simply add more Powder D to the load and move up the red line into the efficient burn zone, but then the bullet speed would also increase.

As I mentioned, the effort of changing powders is made easy within certain powder families. If Powder D represents N340, then moving to N330 or N320 is an easy way to make an incremental change. If you are using Win231 and move to TiteGroup, then your work is probably going to be more difficult, but it can be done.

Hope this helps. 

[Lemming]

Nice.

I guess you could also extend your analysis on the other side of the graph,  where a powder may be too fast to make your desired velocity at a safe pressure (although I guess lots of people go at high speed with very fast powders)

[Wobbly]

I was actually considering such. Looking at it, I think the best way to do that might be to angle the A, B, C, D graphs slightly different from each other. 'A' should be more vertical, while 'D' should maybe lean to the right slightly more. This would maybe give a mental picture of faster powders having a narrower "window", while at the same time as Power A being "too hot" for the application as you suggest. Or the line of constant bullet speed could be a curve itself.

I need to think of some of the implications first, great suggestion though.

 

[nicky]

     Very clear explanation. Even I was able to understand it. Gives me a bit to think about & try.
     

[Towns]

Great graph Wobbly.  I'm greenhorn enough that I certainly can't poke any holes in it.  But I do get what you are saying. 

And I have seen it happen with my loading:  I was getting black sooty cases and brass not ejecting properly while using CFE pistol in my .380.  I changed to Titegroup and obtained the same velocity along with good ejection and clean brass.