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What does soot on fired cases mean? Not expanded to fit the chamber because of low pressure? If so, what is the magic pressure number that is needed to get the necessary expansion? I just had that experience today using a new (milder) load in my Mosin Nagant.
 

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The magic number depends on how hard and thick the brass is. The pressure seems to be about 35,000 PSI - just a little under 30-30 pressure with infantry rounds. If the brass has a good anneal it will seal off a good bit lower.

Thin brass like a 45-70 is maybe 20,000 PSI if it has been annealed
 

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Fritz, I find that soot can also be caused by poor bullet retention, slow powder or weak primer, particularly in a generous military chamber.

If the bullet pops out of the case before the neck expands and seals, sooting will occur.

If you want to shoot lighter loads, you could try to seat a bit deeper, faster powder or apply a crimp.

Daniel39
 
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