Carbon/soot on Case Shoulders

[lawrence97]

I have exhausted all attempts at trying to solve the soot on shoulder issue. I am getting the soot on Alpha 6BRA brass as well as on my 284 Winchester using Lapua 284W brass. I typically use a .268 neck on my 6mm’s and a .313 on my 7mm’s and never experienced this.

The problem exists on my NO turn chambers and what is odd is that the virgin brass seals just fine. The soot appears after resizing on the second firing.

Most would say:

1-Your running low pressure but not in my case. My 6bra is running 2900-2920 using Varget/h4895 and Sierra 107 or Berger 108’s. My 284w w a 28” barrel is pushing Berger 180’s 2760-2790 using h4350 and h4831sc.

2- You have excessive headspace and are bumping the shoulders too much…. Well I only bump .002-.003 as measured with firing pin removed and to allow the bolt to drop closed freely. I did try sizing .005-.001 but I got some resistance on closure and on opening of the bolt and the soot was still present on the shoulder.

3- You need to anneal more aggressively….. I got my 6bra and 284w brass glowing red at the neck to perform the test and the soot still there.

4- Your die isn’t compatible with your chamber. For the 6bra I used a Harrell’s and a whidden die with the same results. With the 284, the body shoulder junction on virgin brass measures .473”, fired brass is .478” and resized is .476”.

5- You have insufficient neck tension. I used an arbor press with seating pressure gauge. Usually 20-40psi is where I am at with .002 neck tension. I also tried in the 40-60psi range and some of my 284 was registering over 90psi and none of this made a difference.

6- Your chamber neck is over sized. My no turn 6bra is a .274 and my 284 is a .319. My loaded 6bra is .269 and loaded 284 is .314. Virgin brass does not exhibit the soot on the shoulder.

7- I did test some of my brass that was neck turned for a .268 chamber. This brass was .265 loaded and it did seal when I fired it in the .274 no turn chamber. The whole point of the No turn is to not have to neck turn!

As for accuracy at 600 yards both are performing with excellent results in an fclass setting(2.5”-3.4” 15 shot groups and vertical is in the 1”-2.4” range). My chrono numbers with the 284 are nice and low. The 6bra has some higher ES/SD but it’s shooting very good. My .268 6bra had some very tight chrono numbers but it did not shoot any better than my NO turn. For 600 yds I am not too worried about what the chrono is telling me.

Any suggestions or something I might be overlooking?

 

[Bill Norris]

FOLLOWING

 

[bugman]

here are a few suggestions that are worth the time.....
Scope the neck-shoulder junction of the chamber. Your fired cases look rounded, compared to the sharp shoulder-body junction. It is also worth inverting the bushing if it has a prominent bevel. Don't worry about the slight tension difference at this time.

shoot several cases that are laddered with charge weights that result in OBVIOUS pressure. If you have to, move to a faster burn rate powder, don't worry about accuracy, and evaluate the difference between a few cases that were really stepped on.

The results of these 2 steps will put more light on your mystery.

By the way, using a 6BRA to draw conclusions about this rifle is really apples to oranges

 

[whatsupdoc]

I had a similar issue with a 6mm Creed PRS rifle, carbon buildup at the edge of the neck area was the
culprit.

Was not that much either.

 

[lawrence97]

I had a similar issue with a 6mm Creed PRS rifle, carbon buildup at the edge of the neck area was the
culprit.

Was not that much either.

I could see that being a problem if you neglected cleaning and don’t use a borescope. I clean my rigs every 80-100rds since that’s what they would typically see during an fclass match and I verify my work w a borescope.

 

[lawrence97]

here are a few suggestions that are worth the time.....
Scope the neck-shoulder junction of the chamber. Your fired cases look rounded, compared to the sharp shoulder-body junction. It is also worth inverting the bushing if it has a prominent bevel. Don't worry about the slight tension difference at this time.

shoot several cases that are laddered with charge weights that result in OBVIOUS pressure. If you have to, move to a faster burn rate powder, don't worry about accuracy, and evaluate the difference between a few cases that were really stepped on.

The results of these 2 steps will put more light on your mystery.

By the way, using a 6BRA to draw conclusions about this rifle is really apples to oranges

The camera angle and soot on the shoulder make the cases appear to be different.

I have taken the 6bra up to 3050fps and I don’t think my brass would appreciate it if I kept on running it there. My .268 neck tuned around the same area and did not exhibit soot.

I’ve used Redding and harrells bushings and no difference.

The 6bra vs 284w is apples to apples since they are both NO Turn chambers exhibiting the same issue which is soot on shoulder and both are running ideal and well known powders and bullet combos per caliber.

 

[bugman]

what do you have against stepping on two or three cases at this point? Once you are overpressured, properly annealed and sized brass will seal at the NS junction and you will not have that blowby. Then, take that overpressured brass and base the headspace bump off those dimensions. At the very least, those 2 or 3pieces will tell you if you are really not undercharged or oversized.
You should ask yourself......If the BRA is so similar to that rifle, then why aren't the powder choices, burn rate, bullet weight, and tension range the same?

 

[lawrence97]

what do you have against stepping on two or three cases at this point? Once you are overpressured, properly annealed and sized brass will seal at the NS junction and you will not have that blowby. Then, take that overpressured brass and base the headspace bump off those dimensions. At the very least, those 2 or 3pieces will tell you if you are really not undercharged
You should ask yourself......If the BRA is so similar to that rifle, then why aren't the powder choices, burn rate, bullet weight, and tension range the same?

Bugman,
Thanks for your troubleshooting advice. I have already run more than just a few cases beyond normal and safe pressures and there was still soot on the shoulders.
Why does running my same load in virgin brass not produce soot on shoulder?

I never said both cartridges were similar or alike. I only said they were apples to apples due to the fact they are NO turn chambers. I have also run both with light and heavy neck tension and soot was still present.

 

[bugman]

Why does running my same load in virgin brass not produce soot on shoulder?...the usual reason is because the virgin brass is much softer and the NS junction seals completely upon ignition, if the FF charge is sufficient.

Anyway, you might want to consider a different lot of brass for comparison sake.

Carry on.

 

[lawrence97]

Why does running my same load in virgin brass not produce soot on shoulder?...the usual reason is because the virgin brass is much softer and the NS junction seals completely upon ignition, if the FF charge is sufficient.

Anyway, you might want to consider a different lot of brass for comparison sake.

Carry on.

The virgin brass being softer is my thought also but I took a few pieces of brass and annealed to a glowing red neck and those still had soot on the shoulder. I would think annealing that much would have made it as soft or softer than the virgin brass ever was.

 

[fkimble@charter.net]

Per your numbers, both caliper necks are no turn and your loaded neck O.D. to chamber neck I.D. is .005 difference. Your .268 neck barrel was loaded O.D. of .265, only .003 difference. Most no turn 6BR types necks are .271 or .272. Maybe that extra .002 clearance allows a little pressure leakage before it makes a good seal. How much jump does your bullet have, or another way of looking at it is how much bullet is seated in the neck?

You said factory new brass did not exhibit the soot. Maybe the factory shoulder is a better fit headspace wise than your resized brass, giving a better seal. Have you checked OAL? Against your chamber, not some reloading manual trim to length?

You said your shoulder set back was based on bolt closure? You threw out a number but I'm not sure if it was a real measured number or an estimated number.

The pic of two cases appear to have a difference in body to shoulder and shoulder to neck length and shoulder sharpness is different

The second pic appears to have a well defined cut off of soot on the lower section of shoulder and no soot on the neck.

Have you confirmed your scales accuracy? Have you confirmed bullet diameter? I doubt your having two calibers with under sized bullets.

Last item, I promise. The turned brass did seal when fired in the no turn chamber? Maybe the thinner brass expanded better/quicker causing a better seal. Have toy tried Lapua or another brand of brass?

Please keep us informed

Frank

 

[mikecr]

Were the trimmed lengths different?

 

[lawrence97]

I thought you were using an AMP. If you are using a torch and got it as described, you may have overannealed. Many similar stories about brass that is too soft not being able to resist the blowby pressure. The easiest quantifiable way to measure effective torch annealing is tempilaq, or similar.
At what number of firings did blowby start to appear?

On the second firing it’s appearing. I only went to the extreme of annealing to see if there was a difference vs my regular annealing. Annealing normally, excessively or not at all has. O effect on soot.

 

[lawrence97]

Were the trimmed lengths different?

Nope. I have only chamfered and deburred. Most of the brass is barely on the second firing and very consistent in oal.
I have heard of those that have trimmed their oal excessively and run into problems. I’m usually within 10 thousandths of chamber max oal.

 

[Old Navy]

Try backing your bushing off .100" or so and see if that gives you a seal.

 

[lawrence97]

Try backing your bushing off .100" or so and see if that gives you a seal.

I did try that. I used my harrells die with an o-ring as a spacer. The bushing sized about half or maybe a tad more of the neck. It’s was better as far as soot but the bullet seating process felt odd once the bearing surface of the bullet got passed the sized portion of the neck. Accuracy was not the greatest at 600 either.

 

[Walt Krafft]

If your load and rifle are shooting ok, what difference does it make if there is soot on the shoulder or not?

 

[lawrence97]

If your load and rifle are shooting ok, what difference does it make if there is soot on the shoulder or not?

Not a whole lot. Just checking if there’s something I might have overlooked to get a handle on the soot.

 

[Redhawk]

Lawrence, a different angle…. How do you clean your chamber? Any carbon build up in the chamber? Any wet residue remaining after the clean in the chamber? How do you clean the brass? Anything on it that carbonises on the case? Some long shots there……

An observation would be that the shoulder angles in that first pic looks very different to each other…..

Sorry, one more….. can you drop a projectile into the neck of a fired case? Or is it a tight fit?

 

[chop house]

Any suggestions or something I might be overlooking?

i only speak from my mistakes, so.... i had length trimmed some brass and deburred the neck with one of those cheesy lee thimble tools. i got sloppy, or the tool quit cutting cleanly, and i pushed down a burr that really could only be seen under a magnifying glass. upon firing the burr flattened out but.... it was not fully around the neck, being more pronounced on one side usually.

i posted a photo quite some time back and i think it was on a very similar thread as yours.