Sooty Shoulders - What's causing this?

[Pinnah]

Hey folks,

I'm getting soot blowback on my brass and seeking guidance on what to do to get it to stop (on the assumption that it is a problem).

Gun: Swedish Mauser (6.5x55) Carl Gustauf M96 (1912)
Powder: 45.4 grains of IMR-4831 (the Hodgdon data is 43.0 - 47.0c)
Primer: CCI Large Primer
Bullet: Hornady Interlock 129 gr seated at cannelure (COAL 2.920)
Brass: PPU with 3 firings (2 since annealing)



I fired 8 shots today, all with this load. The last 3 shots produced these soot patterns.

I've seen these before and with very light loads of H4831, and have even gotten dented shoulders. I assume this is a sign that the case necks aren't creating a good seal. Should I...

1. Move to a faster powder like H4350?
2. Move to a magnum primer when using IMR-4831?
3. Re-anneal the necks and see if the problem goes away?
4. Stop worrying about it?

Thanks for any help you can offer.

 

[Fast14riot]

Typically soot on the necks is either too low of pressure or work hardened brass.

 

[mikecr]

I take it you're crimping necks. Ideas:
High chamber end clearance?
Excess neck clearance?
Too slow of burn/low pressure from powder?

Like you said, too slow to seal

 

[Pinnah]

I take it you're crimping necks. Ideas:
High chamber end clearance?
Excess neck clearance?
Too slow of burn/low pressure from powder?

Like you said, too slow to seal

Could you describe what you mean by the ones that are bolded?

These guns have a very long distance to the land, as they were designed for 160 RN bullets and most modern bullets are in the 120 to 140 gr range.

Cases are trimmed to spec (as much as one can with Lee trimmers). Is that what you mean by neck clearance? Or do you mean case thickness at the neck?

Thanks!

 

[joshb]

Congrats for your appreciation of a fine old gun! I’ve found H100V to work very well in my 4 old Swedes. It’s a little faster burning than the 4831 and might solve your problem. It’s not a popular powder, so picking up a pound may be easy.
I would also suggest you anneal after every firing. I do.
Good luck!

 

[Straightshooter1]

Could you describe what you mean by the ones that are bolded?

These guns have a very long distance to the land, as they were designed for 160 RN bullets and most modern bullets are in the 120 to 140 gr range.

Cases are trimmed to spec (as much as one can with Lee trimmers). Is that what you mean by neck clearance? Or do you mean case thickness at the neck?

Thanks!

Based on the numbers as to how you're loading those cartridges, it's not a low pressure issue. If you haven't ever annealed them before, it could be those cases are work hardened from what looks to me like pressure that's just above SAAMI max. So, I'd be sure to anneal them and take take some measurements on the fired cases and your chamber to see how those cartridges fit so to get an idea what your clearances are like.

 

[DGP4]

The way the soot extends down the cartridge, I suspect you may have an eroded throat. I'd have the chamber cast and see what it measures. You may be in line for a new barrel.

 

[KMart]

High chamber end clearance?
Excess neck clearance?

I am going to try to give you some numbers to compare.
On the 6.5x55 CIP reamer drawing, the neck diameter is .301 and a freebore diameter of .2693
On a 6.5x47 L, the reamer drawing, the neck diameter is .293 and the freebore diameter of .2646

Your X55 rifle has an excess neck diameter of .008 and a freebore diameter .005 larger than your bullet.. That gives a lot of loose area at the front of the cartridge and the cartridge case has a long way to go to swell and seal everything up.
Why the large numbers on the X55? Because it is a battle rifle and had to operate in muddy nasty conditions. They were also designed to shoot a very long 160 grain round nose bullet that took up a lot of that space.

 

[243winxb]

https://apps.dtic.mil/docs/citations/ADA622138

Increase neck tension/bullet hold, so bullets don't move to soon. A faster powder or magnum primer may help. The long jump to the rifling, makes it worse.

The primer is capable of producing enough force to
debullet the projectile and begin the engraving process prior to any significant propellant burning.

 

[243winxb]

and have even gotten dented shoulders.

Seem to be happening a lot these day. Using slow powders to try and fill the case to 100% . Light for caliber bullets to get high velocities.
Used to be a problem with big magnums with extra free bore.

https://forum.accurateshooter.com/threads/strange-dent-in-fired-case.4016999/
See photo post #38.

 

[joshb]

Here’s an old pic I found.
PPU brass. 2nd firing , annealed.
Primers were either Federal or CCI
H100V
120 AMAX, no crimp. The only rifle cartridge I ever crimp is 45/70 for my lever gun.
I didn’t record the OAL because the jump is ridiculous.

 

[243winxb]

Here’s an old pic I found.
PPU brass. 2nd firing , annealed.
Primers were either Federal or CCI
H100V
120 AMAX, no crimp. The only rifle cartridge I ever crimp is 45/70 for my lever gun.
I didn’t record the OAL because the jump is ridiculous.
View attachment 1263125

Nice target.

You loads look very light when compared to Hodgdon data. H100V looks like a double based ball powder containing Nitroglycerin.
The OP's IMR 4831 is single base, no Nitroglycerin.

What are you doing to avoid soot?

 

[joshb]

Nice target.

You loads look very light when compared to Hodgdon data. H100V looks like a double based ball powder containing Nitroglycerin.
The OP's IMR 4831 is single base, no Nitroglycerin.

What are you doing to avoid soot?

Not an expert but I think it’s the faster powder and the annealing. I always anneal after every firing. I shoot the M38, 2 1896s and a CG-63 and I’ve never had a soot problem.

I usually shoot lighter loads in the small ring Mausers , even tho the Swedes are supposed to be better steel.

 

[243winxb]

H100V looks like it has a filling.

 

[Steveb]

Very interesting read and report. I have had exactly the same issue in my 6.5 x 284's using a 162 grain bullet. Re23 and BR2 primers in the last months. My gunsmith made an error in the length of the freebore and I need to seat the bullet almost to the end of the neck to get close to the lands. I started loading using a .010" jump using a fairly medium/normal pressure load of Re23 and CCI 250's for a primer initiating test. The bullet was only being held about .125" into the neck. The first shot blew the primer completely out of the case yet the case was completely sooty indicating low pressure. After testing other powder loads and further discussion we felt the bullets were being forced out of the case by primer ignition due to little neck tension and short bullet travel before most of the powder ignited. We felt that when the bullet was out of the case, some of the burning/burnt powder flowed down the outside of the case causing the soot then the main powder charge ignited possibly causing a high pressure spike. So we set the bullets back into the case for .386" jump. We choose this setting because a lighter bullet using this OAL wasn't having any issues. The next 4 shots performed perfect as a reduced load should with no soot and no pressure. My conclusions last week from the tests was that when the bullet base can clear the case before the powder ignites you can cause pressure and weird issues. This report from the US Army seems to back our theories out.

Your 6.5 Swede with short bullets and the long military throat duplicates my setup to some extent and you seem to getting the same issues we are.

Steve

 

[dennisinaz]

Most likely you have a badly erroded throat. Throwing a new powder at it won't fix problem. A very long bullet might help, like a 160 Norma

 

[lotech]

If your load is truly light and you want to stick with the powder you are using rather than going to a faster powder, increase the powder charge. You're not getting sufficient pressure to expand the case necks against the chamber to seal against powder blowback. You can try a hotter primer, but in most instances this won't be sufficient to alleviate the problem. An eroded throat is a possible cause, but it would have to be severely eroded to cause gas blowback.

 

[Pinnah]

High chamber end clearance?
Excess neck clearance?

I am going to try to give you some numbers to compare.
On the 6.5x55 CIP reamer drawing, the neck diameter is .301 and a freebore diameter of .2693
On a 6.5x47 L, the reamer drawing, the neck diameter is .293 and the freebore diameter of .2646

Your X55 rifle has an excess neck diameter of .008 and a freebore diameter .005 larger than your bullet.. That gives a lot of loose area at the front of the cartridge and the cartridge case has a long way to go to swell and seal everything up.
Why the large numbers on the X55? Because it is a battle rifle and had to operate in muddy nasty conditions. They were also designed to shoot a very long 160 grain round nose bullet that took up a lot of that space.

This helps a great deal. Thank you.

Given this, what would you suggest to eliminate the sooting? Faster powder? Increasing the charge? (I'm still inside of Hodgdon's data but at or above others).

 

[dellet]

Since it only happened on the last three case, and is a very consistent pattern, I would carefully measure the and compare the cases with the soot and without. I would be looking for a flat spot or crease. Then look for a piece of carbon or other garbage in the chamber. I would expect a pressure only problem to be more random.

Next time shooting I would index the cases to find out if it is indeed happening in the same spot of the chamber.

Curing the problem would simply require a bushing neck die, full length if you prefer and can get one. This could be set to leave the base of the neck formed to the chamber by only partially sizing.

 

[joshb]

Reading thru the last few posts, I agree with the thought of adding more neck tension, either with a new smaller bushing or with a crimp, to slow the bullet release and allow more pressure to build. Using a faster powder might help do that but in these times, finding that powder may be impossible.
The main reason for this post is to advise caution when increasing powder charge in these old guns. They weren’t designed to shoot at modern pressures. My Hornady book has this advise, which I will include.
I’m in the process of building a 6.5-55, using a Stiller Tac 30 action and a Rock Creek Barrel. I want to explore the potential of the cartridge at higher pressures that I don’t dare load in my antique Swedes.


Oh, and I found my recorded COL!