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Going to all be guessing as home-brew pressure readings are pretty much like reading tea leaves...if there has been published data on the amount of pressure it takes to expand brass to a seal, I missed it.

Have to take into account the hardness of the brass, the number of times it's been cycled through loading (it does become harder...less ductile....at each firing, and the more the brass is worked by the die, the harder it becomes) and the diameter of the chamber in which it is fired (with larger being harder to seal than smaller).

HAve to take into account the burning speed of the powder...if gas gets between the case and the chember early, as pressure rises, the case still won't seal (once open...the system just won't close easy...some might fast .240WM loads still tend to collape the case shoulder and come out sooty).

ON a WAG, from lodaing a lot of mouse-poot loads, I'd think that something in the order of 11K-12K for .45Colt cases to form a non-leaking seal..with brand new cases perhaps taking 10% less. Find that most .22CB caps doen't seal all that well, but that case is thinner and the pressure level is pretty low. About 1/2 the factory .32SW (the short one,,,not the .32SWL) come out sooty.
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Know it's not what you asked, but when working with very lite-loadsm prefer to use fired cases that are only sized enough to enter the chamber. Sooty, non-sealing, cases are not just an aggrivation, with light loads it introduces a variable...some seal, some don't, so some lose pressure to the rear and others don't....that makes lite-load accurcy hard to come by.
 

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Spend a lot of time loading mouse-poot loads for big rifles so I can shoot off hand at an indoor range in foul weather. Use some small centerfire rifle rounds as well for the same practice (.32-20 / 9mm Largo / 38-40) and some antique rifles (45-70 / 50-70). Just seems that if my graph of assumed pressure is right, things get "iffy" at 10K. If I have thin soft new brass, they form to the chamber and don't soot up the case or blow gass into my face at about this level. Old brass (that has grown harder) or brass that is made a bit thick at the start takes more...the 9mm Largo "Destroyer" Carbine is a good example. Takes at least 14K to get those cases to complete seal.

Some rounds, even with new brass, are really bad about this. Have a .401WSL I hunt with, but it takes at least 16-20K to seal those cases (they are THICK)...and no slow powders need apply.
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Good advice to steer clear of any relaod that you didn't pull the handle on. Better to spend the $ on either factory loads or new brass you reload yourself. Can still screw up, but at least you'll know who to blame.
 
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