The theory behind brass getting powder burn on the outside

[PlinkERDoc]

I have been trying to figure out why some loads are causing powder burn on the outside of the case. I have a theory and am looking for the experienced folks on this forum to let me know if the theory is right or wrong and, if it is wrong, what is driving powder burn on the outside.

Theory of powder burn on the outside of the brass casing

I am using the simple blowback operation but believe the same applies to delayed blowback operation. It goes something like this:

• When the powder starts burning, the pressure inside the case increases, resulting in three things:
  • the bullet is pushed forward,
  • the bolt is pushed back,
  • the brass is expanded to seal the barrel, preventing gasses from escaping past the seal
• While the bullet travels down the barrel, the expanding gasses create pressure that pushes the bolt back, pulling the case along.
• As long as brass is still inside the chamber, the seal is maintained, and gas pressure pushes the bullet forward.
• When the bullet exits the barrel, all gasses escape through the muzzle end.
• If the case is pulled from the chamber before the bullet exits the barrel, the gasses exit through the back of the chamber, around the case, causing burns on the outside of the case.

Does this make sense at all? Am I missing something?

 

[67ray]

What gun is this happening on?

 

[Uzi2]

Happens mostly with slower burning powders and light loads. The initial pressure spike isn't enough to seal the brass against the chamber wall so there's gas leakage into that space.

 

[Fritz the Cat]

100%

 

[gerrycaruso]

I thought it was caused by loads that are too light to create a complete seal.

 

[PlinkERDoc]

Happens mostly with slower burning powders and light loads. The initial pressure spike isn't enough to seal the brass against the chamber wall so there's gas leakage into that space.

Thank you for the pointer!

So my theory on powder burn rate being the cause is not valid. Do you know if there is a way to figure out if the powder is too slow or too fast for a given gun?

 

[Rob Boudrie]

Nice longitudinal stripes on brass from HKs with fluted chambers.

 

[Quite_Exasperated]

Maybe the chamber is out of spec? Perhaps not the ammo at all?

 

[meh]

I have been trying to figure out why some loads are causing powder burn on the outside of the case. I have a theory and am looking for the experienced folks on this forum to let me know if the theory is right or wrong and, if it is wrong, what is driving powder burn on the outside.

Theory of powder burn on the outside of the brass casing

I am using the simple blowback operation but believe the same applies to delayed blowback operation. It goes something like this:

• When the powder starts burning, the pressure inside the case increases, resulting in three things:
  • the bullet is pushed forward,
  • the bolt is pushed back,
  • the brass is expanded to seal the barrel, preventing gasses from escaping past the seal
• While the bullet travels down the barrel, the expanding gasses create pressure that pushes the bolt back, pulling the case along.
• As long as brass is still inside the chamber, the seal is maintained, and gas pressure pushes the bullet forward.
• When the bullet exits the barrel, all gasses escape through the muzzle end.
• If the case is pulled from the chamber before the bullet exits the barrel, the gasses exit through the back of the chamber, around the case, causing burns on the outside of the case.

Does this make sense at all? Am I missing something?

We see it for different reasons in different cases. For example, you'll see it with standard pressure 45 Colt loads in revolvers and lever guns because the brass is thick enough at the mouth that there's not enough pressure to force the brass to obturate. That's one of the reasons why people like 44-40 in lever guns instead--very thin case mouth = better seal.

With blow-back semi-autos, yeah, I think the problem is that there's no delay, so the casing starts to be pulled out while there's still a lot of hot, dirty gas in the barrel. 9mm isn't a perfectly straight wall, so though the case mouth obturates early on, as soon as the bolt starts pulling it out of the chamber, the seal is lost immediately.

 

[gerrycaruso]

When you say "too slow or too fast for a given gun" do you mean rifle vs handgun?

 

[vicorjh]

Do you know if there is a way to figure out if the powder is too slow or too fast for a given gun?

Beyond using the reloading books, If you are experimenting, Quickload is a software tool that can help. It will produce pressure / velocity graphs for your given load data and gun configuration.

 

[milktree]

If the case is pulled from the chamber before the bullet exits the barrel,

This part is 100% wrong.

The bullet is traveling way too fast for this to happen. If it did, you’d have hot high pressure gas squirting out of the breech and the whole gun would be filthy, and the case would rupture. The unsupported mouth of the case isn’t nearly strong enough to withstand the pressure of the burning powder.

 

[SJan]

From my experience, I don't think it has anything to do with the semi auto boltor slide cycling. I say this because when I shoot my light loaded 38 special in my marlin lever gun, every case has the powder residue on the outside of the case.

I have seen this with 9mm loaded with a starting charge of a very fast powder

 

[Mountain]

+1 what others have already stated: Loads are too light, resulting in insufficient pressure that causes the 'leaks' onto the outside of the case. Also, as others have stated that case is moving too slowly relative to the bullet for the issue to be a case exiting the chamber too quickly. Check out the slo-mo.


View: https://youtu.be/HElQk2wEY5w
Turn down the sound or be subjected to overly pretentious wedding music.

 

[PlinkERDoc]

Thank you, everyone, for your explanations! This is great! I learned something new.

 

[PlinkERDoc]

When you say "too slow or too fast for a given gun" do you mean rifle vs handgun?

I was thinking about handguns but I am interested in figuring out too slow/fast powder burn indications for both(I use bolt guns, mostly).

 

[Rockrivr1]

I

I was thinking about handguns but I am interested in figuring out too slow/fast powder burn indications for both(I use bolt guns, mostly).

These came out of a Uberti 1860 Lever Action rifle. I bought Cowboy Action 45 LC to shoot in this rifle and it was just too weak for it. I got burned brass and blowback into my face, which was not enjoyable. Move up from cowboy action rounds and the rifle seals up the round properly.

 

[meh]

This part is 100% wrong.

The bullet is traveling way too fast for this to happen. If it did, you’d have hot high pressure gas squirting out of the breech and the whole gun would be filthy, and the case would rupture. The unsupported mouth of the case isn’t nearly strong enough to withstand the pressure of the burning powder.

Yes, of course the case is not going to be pulled before the bullet gets away, but there's a difference between what's happening in a recoil-operated semi-auto pistol and a simple blowback-operated PCC without delay. Look at this point in the video of the recoil operated pistol. With the recoil operation, the bullet is gone at this point, the slide has moved noticeably, but extraction has not yet begun. With a simple blowback PCC, the bolt will have moved less at this point because it has more inertia, but will still have moved, and there's obviously still a positive pressure differential with hot gas and the products of combustion in the barrel not having completely vented. That's why a simple blowback PCC is dirty: the seal at the case mouth gets broken before venting is complete.

 

[gerrycaruso]

A burn rate chart would probably be a good place to start. The reference section of a reloading manual and the manufacturers description might also be helpful in determining suitability. If you're new to reloading, a class where you could discuss this would likely answer your questions.

 

[pastera]

Thank you for the pointer!

So my theory on powder burn rate being the cause is not valid. Do you know if there is a way to figure out if the powder is too slow or too fast for a given gun?

Too fast of a powder will spike the pressure at low charge weights well before you get to a decent velocity.
Too slow will result in low velocity even with a full case.

Some soot on the case mouth is normal
Excess soot on a factory load could be a low power target load (read cheap) or an oversized chamber (likely for a blowback)

 

[PlinkERDoc]

A burn rate chart would probably be a good place to start. The reference section of a reloading manual and the manufacturers description might also be helpful in determining suitability. If you're new to reloading, a class where you could discuss this would likely answer your questions.

The charts and manuals are a good starting point, but they assume standard variables, like barrel length. For example, a 9mm cartridge is used in a 3.5" barrel, vs. an 18" PCC barrel might need a different powder charge or burn rate.

 

[Road_Clam]

Agree 100% with others, weak load (pressure) causes insufficient casing expansion to seal against the chamber. You end up with gas blow-by and sooty brass . I saw this first hand loading light 38 special in my 357 mag GP100. Wicked sooty casings. Went back to loading full bang 357 mag and brass looked much cleaner overall.