Soot on shoulder and pressure signs(?)

I’ve been loading 6BRA (Wheeler design w .271 neck) for a few months. I’ve been surprised at the low amount of powder required to show pressure.

These cases are a random sample of my last firing. Note the extractor marks, raised primers, and soot all the way down onto the shoulder.

Specs: necks skimmed to .011/side for a loaded diameter of .2680/.2685. This batch was 29.1 to 30.1gr of H4895 behind Roy Hunter 103s at .006 jam. Max velocity in the 2920s. BR-4 primers. AMP Annealed two firings ago. Whidden die, .0015 shoulder bump. Neck is .2700/.2705 after firing. Barrel was chambered by a well regarded smith.

At 30.0gr of H4895, the primer pockets slowly get loose. Not bad but definitely at my upper limit. Bolt lifted with a little resistance.

Fire-formed brass has clean shoulders.

Question 1: Are these velocities and powder charges in the normal range? I don’t mind if my barrel is slower than most but accuracy is only so-so.

Question 2: The shoulder soot can’t be good, what should I do to eliminate it?

Thanks, David

 

My non-expert but experienced mind says:

1. stop jamming them, go to .020 jump and test, see what you get. I never jam unless I absolutely have to, it's nothing but problems. Betcha your primer pockets stop loosening up if you go to a short jump from the jam and I'll bet your sooting issue moves to the necks from the shoulder.
2. Sooting to me suggests that the necks aren't expanding quick enough to seal the chamber so might want to anneal every time too.

 

"2. Sooting to me suggests that the necks aren't expanding quick enough to seal the chamber so might want to anneal every time too."
This has been seen in my (limited) experience as well. Soot on the shoulder can mean that the neck is not expanding as quickly as it should/could. A few not quite right annealings with the torch seems to be the main culprit in my case. Wish I could afford the AMP...
Could the jammed seating depth also be playing a part? The gasses have no where to go initially so just blows around the neck before it even has a chance to expand?

 

Possibly. I have played with anywhere from large amounts of jump to jam and so far the only accurate node is with jam. My last session was all jam and don't remember seeing this kind of carbon before now. I could anneal every firing instead of every third, but that could be ignoring the root problem.

 

0.011 per side (+/- 0.2655) or 0.268" loaded?

Barrel length?
Bore Diameter?
Shoulder set back by die clicking or measured bolt drop on sized brass?

Charge seems appropriate for those velocities, but the speed is comparably low.

The explanation of not sealing fast enough sounds reasonable enough, but not sure how useful it is if you're already wearing out primer pockets with little room to explore higher charges.

Where you live (altitude, humidity, etc.) can influence where your gun will run comfortably and tune. A good example of "what one famous guy did at a specific location with a specific powder and bullet" not being applicable everywhere.

 

dgeesaman u stated above u had (Specs: necks skimmed to .011/side for a loaded diameter of .2680/.2685.) U may want to double check the math there. If u have cal. .243 plus .011 x 2 that = .265 loaded neck dia. No pun intended.

 

Precisely what I'm betting and why I suggested backing off to a jump from a jam.

 

I shot a lot of hunters 103 "bullet a" this summer and did pretty well with them anywhere from ~.006-.016 jam. And 29.6 gr of h4895 and federal primers shot ~2920 fps. Mine seal up right below where the bushing stops sizing and its consistent from case to case. I anneal every time though, and harder than most.

Did you measure loaded round to be .268? Or just assuming from neck turning to .011?

Edit: Which as stated above would seem to be close to .265

 

Yeah the math doesn’t add up. Loaded is definitely .2680/.2685 so I will double check my neck wall measurement.

My bushing is currently sizing almost the entire neck - perhaps I should back it out .06 or .10” and size a bit less?

David

 

Not saying this is the case, but I have seen what is considered to be a low pressure load exhibit classic signs of what high/over pressure loads do.
Playing with cast bullets, jam is done on a fair amount of loads. Then a small dose of fast burning powder(which I disagree with myself) doesn’t let the brass do its job. More so if it has been work hardened,old,found on the range for free, etc. A proper anneal goes a long way. CB shooters tend to be cheap for the most part. End results more times than not, you get no better than what you start with.

I was shown a 700 in 30-06 in pieces. Soot clear back to the extractor groove and flat primer. Same load I see repeatedly doted and quoted from the late 30’s to present day.

Once again not saying this is the problem as far as low pressure. But if one was to err on the low side it can be a possibility.

 

The fact you're using a bushing die, you cant size the entire neck anyways. I'm only sizing about .180" or so of the case. Just enough to hold the pressure ring.

I'm not sure that's your issue with the clearance you have.

What number did you hit them with on the amp ?

 

Are you measuring loaded diameter with a caliper? Take a fired and try to seat a bullet with your fingers. Does it go in easily?

 

I have 2 batches of Lapua brass for the same rifle. It has a no turn chamber, but I still wanted to turn for cleaning up neck sake. One batch i turned the necks the other no. The ones I turned and created extra neck clearance and get soot all over the neck and shoulder, the ones that don’t have turned necks don’t get soot.

 

Loaded diameter measured with a micrometer. I checked neck walls with my tubing mic and got .0120 to .0125. Brain fart. So the numbers go with the .268 loaded diameter.

A bullet does pass into a fired case.

My AMP has the Aztec mode and I don’t melt a case each time but I use code 0152 based on several samples with three firings on them.

My neck bushing is set as deep as it can go and so it’s sizing 95% of the neck length. Sizing less neck and leaving a step seems like a very good idea.

Neck tension is currently based on a .265 bushing followed by a .2415 expander mandrel.

David

 

Either you have too much neck clearance or you need more neck tension. That .2415 expander is barely giving you .002" neck tension. I would also try another powder before you beat yourself up anymore about low velocity in that barrel. Some barrels do not produce with certain powders.

 

A .2415 expander on annealed cases should give you right around 8-10# seating pressure. Myself, along with many of the top Williamsport shooters use the same expander.

 

I got sooty shoulders in 6BR and 6.5x47L when first using my custom Whidden dies. You can really oversize with them (by design). Mine are not bushing type, rather custom machined neck diameters. I've abandoned trying to measure shoulder bump with calipers and a comparator. I only use the stripped bolt method now, coming down slowly until the bolt falls closed on 5 consecutive cases, plus just a touch more. Brass comes out much cleaner doing it this way.

 

The main thing I watch for is ejector marks on the cases. Back the your loads down. How close you are seating to the lands?, Back it off if necessary to .010" to .025". Another thing that will raise pressure is the case volume. They can vary quite a bit from lot to lot. I load .208 grain bullets close to the upper limit. In my setup, 42.5g of Varget works. I had to retire those cases and got a new batch of Hornady cases. All prepped the same. My groups went for good to horrible. The velocity dropped about 45 fps. I checked some of the cases and they were about 7% larger in capacity. I had to bump the loads up to 43.0 grains to get the velocity and accuracy back. Going backwards, if I ended up with 7% smaller cases, there definitely would have been an over-pressure condition. I had severe overpressure and was well under the maximum load on Hodgdon's site. The max was 45.9g. I wasn't watching at the time but at 44.0 I started getting signs. A little more at 44.5 and at 45.0, it kicked like a mule, I had to pound the bolt back and the case expanded so much the primer fell out. Glad I still have my eyebrows.

 

Tony Boyers book the (The Book of Rifle Accuracy" p. 149 talks about soot on the necks. I won't attemp to interpret it. He relates some of the appearance on 2 pix to necks that are to thin. He also states that the pictures don't relate to too thick of necks. Your pix seem to show soot on the shoulder. No obvious soot on the neck. Regardless of published loads ejector marks are definite high pressure. The primers not being flattened does not mean you don't have pressure. The only way you get ejector marks is when the pressure is so high that the brass gets pushed into the ejector hole and rotating the bolt attempts to shear the high spot off on the edge of the hole. Do the bullets have a pressure ring? Measure loaded neck diameter at the back of the neck.

 

Using my Whidden comparator and brass that's expanded to varying degrees (of course some of them caused some bolt lift force), I get very good correlation between the length of fired brass that causes light resistance on bolt close and the number that consistently closes with ease. I can double-check and back out my die a couple thou and make sure that the resistance is isolated to the shoulder length. I have done this on more than one occasion and derived the same comparator measurement.

I've only used one blue box of Lapua since I started with 6BRA, but I will keep an eye on that since I just cracked open a second box and it's a different lot.

I'm definitely backing down on charges going forward. I had some inclination that 6BRA with 103's would put me in the 2900-2950 range, but clearly I need to stay under 2900 (or lower!) to stay out of pressure signs.