black soot on case necks

[nnn66]

I'm hoping some of you guys may be able to help me understand what is happening and what I need to do to correct it. I'm working with a new 338 Lapua rifle and have only put 65 rounds through it so far. For the first 55 rounds, I used 2 different powders, N560 and Accurate Magpro. I did not have any black soot on my case necks with either of those powders. However, yesterday I shot 10 rounds loaded with Retumbo and all 10 of them had heavy black soot on the necks.

My rifle is a CZ 550 Magnum HET with a 28 inch barrel. I am using 300 grain Lapua Scenar bullets, Federal match magnum primers, brand new, unfired lapua brass, and so far, I have seated all bullets to 3.681 which is book stated max oal. I have just ordered a bullet comparator and oal gauge, so I intend to start working on my seating depth soon.

I just randomly selected a starting point of 87.0 grains and 89.0 grains of Retumbo. The book stated starting load is 85 and the never exceed load is 94. Could it be that I don't have sufficient pressure with 89 grains of Retumbo? From doing a bit of research, I understand that low pressure might be a cause of this. Also, I have read that overly hard case necks might cause it, but this is brand new lapua brass and I didn't get any soot from other powders I have tried, so I don't think it is hard necks. There were no pressure signs with 89 grains, so I intend to go up with it, but I want to get some feedback from some of you before I proceed.

Could it have anything to do with seating depth? I would think not since I didn't have soot with other powders, seating the bullet at this same depth. Right now, I'm thinking that I just need to add more powder. Am I on the right track or is there something else I'm neglecting to consider?

 

[BuzzBoss915]

So, this is new brass that has not been resized, no neck turning? Symptoms do sound like a low power load? You should mic the neck expansion of all fired cases and see if the retumbo cases are smaller. The data with 89 grains of retumbo, what primer was used? Also, doubtful seating depth has anything to do with it. BB

 

[nnn66]

BB: I just took your advice and measured the case necks. Those shot with N560 and Magpro are consistently measuring .370 outside diameter while those using Retumbo are consistently showing .368. Is that a big enough difference to cause this? I suppose it probably is.

And, yes, this is brand new lapua brass, right out of the box. I am only chamfering the necks very lightly, lubing it, and then loading it.

 

[Iamironman]

And, yes, this is brand new lapua brass, right out of the box. I am only chamfering the necks very lightly, lubing it, and then loading it.

Are the loads with soot on them using virgin brass, or is it the same brass that you've used for the prior firings? You might need to anneal the brass. I'd clean up the brass (you might need to do this by hand) and increase the load of Retumbo a couple grains and see if the soot goes away. If not, anneal a few and try again.

 

[nnn66]

This is all virgin Lapua brand brass. I measure the outside diameter of a neck on an unfired, loaded round and it was .364. After firing with Retumbo, they expand to .368 while they are .370 with N560 and Magpro. I had thought that 89.0 grains of Retumbo would be a pretty stout load, but apparently it's not quite enough. I will experiment with increasing the powder charge and let you guys know how it goes. Could my 28 inch barrel length have anything to do with needing more powder to get enough pressure?

 

[nnn66]

I thought I might bring this thread back to the top for a moment and see if anyone else might be willing to have some input. One thing I've been thinking about today is the fact that I expected 89 grains of Retumbo to be plenty sufficient to generate adequate pressure, yet it seems that perhaps it is not. Being new to using Retumbo, I had done a little searching to see what other people were having success with and it seems that some people are having good results with about 89-91 grains of Retumbo in 338LM. This has me wondering why I would be having signs of low pressure at 89 grains. What are the variables that might cause this? I think I've ruled out a problem with the brass and I don't think seating depth is the issue, so could it have anything to do with barrel length? It seems that much of the info I've looked at is in reference to rifles with a 26 inch barrel. Since my barrel is 28 inches, is it likely that this could be causing me to have low pressure with 89 grains of Retumbo? Any other suggestions? This is the first time I've come across this, so I appreciate any help you guys can provide in helping me to understand.

 

[BuzzBoss915]

The difference between 26" and 28" should not be a factor, and if it is I would be very surprised.

But, in any case, since you used three propellents, unless the Retumbo accuracy was far greater than the others, the simplest thing, while not a solution; is to focus on the other two. As far as I am concerned, it does not take too much of an issue to move me in a different direction and unexplained, mysterious black soot is reason enough to run your ladder tests with the others already tried, or new powders. This is a strange situation and the only time I ever ran into it, I had decided to try 40 grain Sierra bullets designed for the Hornet, but in a 220Swift. My theory was head shots on cats and that's what happened, basically blew brains out the ears without an exit. But, I didn't like the black soot on the necks. Different powder, IMR4831, so nothing to really compare in your situation except for black soot which I attributed to low pressure failing to expand the necks. In any case, a 17MachIV does a better job anyway, and the experiment didn't interest me any further and that might be the message, here? Don't fight it, find something else that works. BB

 

[mudpig]

Several posters here have stated this same issue with Retumbo and they all seem to be shooting the lower powder charges. I've been using Retumbo with Hornady 285gr bullets without this issue, but my loads have been 93 and 94 grains. I think that this powder is a very slow one to build pressure and needs a stout load to burn cleaner. I feel that sufficient pressure is not being met to expand the neck until after the bullet has left the case giving room for this fouling to get to the neck. Just a guess...but I know my higher charged loads are not having the same issue....and accuracy seems to be TOP NOTCH!

 

[nnn66]

I appreciate all of the responses. I have only fired 10 rounds with Retumbo so far, but the group I shot with the 89 grain load looked pretty good, so I'm definitely interested in continuing to experiment with it. Not only that, but I recently purchased 6 pounds of Retumbo, so I almost feel obligated to give it a chance. Having done some more searching, I'm seeing more and more people indicating that they are having good results with around 93 and 94 grains, so I think I might just need to up my powder charge and see how it goes. With the book stated minimum to maximum being 85-94, I haven't even hit the halfway point yet with 89 grains, so I guess I still have lots of room to go up. Unfortunately, I have too much other stuff going on right now and won't get a chance to shoot again until next week, but I will give it another try with more powder and then post the results on this thread. Thanks again for all the feedback!

 

[George63]

the black soot is not always a low pressure load, but how the pressure builds before the neck seals to the chamber - a powder with retardant tries to minimize the initial spike in pressure, maximizing the possible charge / velocity - everything is a compromise, retumbo is well known to give excellent velocity and accuracy in 338LM - if the downside is more cleaning of the brass/ chamber, that is not so bad - if you find it unacceptable, H-1000 would be the likely alternative