Soot on Case base and Bolt face

[DLH_]

I understand that it’s common to get soot on the neck of a case or even down the sides with a low pressure load or too thin necks, however I’ve never experienced it on the base of the case. It almost looks similar to a pierced primer, but the soot is easily removed and no marks on either brass nor bolt face. The brass is new virgin Lapua that has been neck turned to a loaded diameter of 0.288 to fit the .290 neck of my rifle. Using 107 TMK which are sat just below the boat tail, cannot engage the lands as the barrel is throated for 123-130gr bullets. Using Vihtavuori data, I started a ladder test. Both the first shots 38.5gr and 39gr showed the same soot. No pressure signs. In the picture it’s compared to a piece of brass shot on the same day but with 38.5gr of N150 and a 123 Scenar sat to touch the lands. The barrel has fired 20 rounds. I cannot understand why this is happening, I’m not using a low charge, all the brass is sized the same, all primed with GM205M primers. Any suggestions as to what’s causing it? Will it damage my barrel?
Thanks,
Josh.

 

[Uncle Ed]

If your cases were my old 54 Chevy engine it would be called blow-by or leakage past the rings.

"Blow-by occurs when the explosion that occurs in your engine's combustion chamber causes fuel, air and moisture to be forced past the rings into the crankcase. Your engine's rings must maintain an excellent fit in order to contain the pressure.'

Meaning your new virgin cases allowed some leakage past the case until they tightly sealed the chamber.

Also your cases do not look fully formed to the chamber, the shoulders still look rounded.

The left hand case in your photo has more soot marks on the neck and no soot on the base of the case. Meaning that case sealed better when fired with no blow-by.

My guess is low start pressure due to your longer throat and using a lighter shorter bullet.

 

[Zero333]

Is this in a gas gun or bolt gun ?

 

[DLH_]

Is this in a gas gun or bolt gun ?

It’s a full custom bolt gun

 

[DLH_]

If your cases were my old 54 Chevy engine it would be called blow-by or leakage past the rings.

"Blow-by occurs when the explosion that occurs in your engine's combustion chamber causes fuel, air and moisture to be forced past the rings into the crankcase. Your engine's rings must maintain an excellent fit in order to contain the pressure.'

Meaning your new virgin cases allowed some leakage past the case until they tightly sealed the chamber.

Also your cases do not look fully formed to the chamber, the shoulders still look rounded.

The left hand case in your photo has more soot marks on the neck and no soot on the base of the case. Meaning that case sealed better when fired with no blow-by.

My guess is low start pressure due to your longer throat and using a lighter shorter bullet.

That seemed the only logical answer to me too. The Vhit load data is very conservative, I think it considers 52K PSI as the maximum pressure when in fact the Sammi spec for the Lapua is significantly higher. The listed maximum for N150 and 123 Scenar is 37.8gr, I worked up to 39gr where I got a slight ejector mark and a small amount of bolt stick. I considered 38.5gr the maximum in my rifle. In which case, should I keep working up in 0.5gr increments until the problem seems to resolve itself?

 

[DLH_]

I think the throat maybe just be too long for the 107 TMK

 

[gunsandgunsmithing]

Try a faster powder.

 

[shoot4fun]

Virgin brass?
Employ a, I think you call them, moderator at the muzzle? They tend to dirty everything up and, if new brass, back pressure getting around the new and undersized brass could cause your issue.
But just a guess.

 

[DLH_]

Virgin brass?
Employ a, I think you call them, moderator at the muzzle? They tend to dirty everything up and, if new brass, back pressure getting around the new and undersized brass could cause your issue.
But just a guess.

That is what we call them and I am using one. I use one on all my rifles from 22LR to 20 Ferguson Ace to 6.5x284; all sorts and haven’t ever had the problem I’m having now. I’m stumped. I’ll try it without and hopefully it clears things up. Thank you

 

[DLH_]

Thank you for all our replies, I’ve got a few things to try now!

 

[243winxb]

The bullet is moving out of the case to soon. This does not allow pressure to rise correctly.

Increase neck tension, seat bullet deeper into case, shorter COL.

Go to a faster powder.

Change primers. If using standard, try a mag primer. If using a mag primer try a standard primer.

https://thefiringline.com/forums/showthread.php?p=4928827#post4928827

Photos at link.

 

[potatoe]

I had this happen when I left a little lube on the cases

 

[duanepa]

I just had this happen an hour ago. First time for this rifle/6BR. The bullet was seated well short of jam (by mistake)... Powder in use is plenty fast for this application. N133 just my 2 cents

 

[Raythemanroe]

Hit them harder next time..

Ray

 

[DLH_]

So far I’ve tried seating them deeper, 20 thou, 30 thou and 40 thou off the lands. Still with the same problem but it has reduced slightly. I will try magnum primers next with the deeper seating depth and see if that combination solves the problem.

 

[duanepa]

To me, that’s a lot of jump. I’d be headed the other way (closer to the lands or jammin)

 

[mikecr]

What is your chamber end clearance (from case mouths)?

 

[Dusty Stevens]

Jump, primers, bullet lot, all that jazz dont matter. The answer is above- hit them harder. Your load is too low. Theres nothing else to try- go up on your powder

 

[gunsandgunsmithing]

Jump, primers, bullet lot, all that jazz dont matter. The answer is above- hit them harder. Your load is too low. Theres nothing else to try- go up on your powder

Yep, unless I missed something, he had no problems with the same powder and charge, but a heavier bullet.

 

[DLH_]

Thanks again, I’ll give it a try!