noticing a pattern in my shells... 9mm [Archive] - Glock Talk

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samsong
08-22-2010, 22:51
The observation is this - when the cartridges are expelled, they seem to have a powder residue in a semi-circle shape (half-oval) on one part of the case mouth out to the middle of the case. All function is normal - rounds are accurate, cycle the weapon, the slide locks back when I'm keeping my dumb thumbs off the release, primers look great, and fouling appears to be normal (copper jacketed or plated lead rounds on AA#5 and Blazer brass).

If I had to guess, I'd say I was underloading the cartridges and the mouth was not expanding before the burning gases were rushing around to the outside of the case.

I don't have a chrono so I don't know how hot my loads are, but I am meeting published specs.

Ideas?

Zombie Steve
08-22-2010, 23:32
They could be underpowered and the case isn't forming a seal with the chamber, but 9mm headspaces on the case mouth, so one other possibility is that your case mouth isn't perfectly squared off. I geeked out once and trimmed some .45 brass, as I knew it wasn't underpowered, and the issue went away. My chrono numbers were more consistent too.... but for all practical purposes, it's nothing to worry about.

noylj
08-23-2010, 17:30
Possibility:
The case is too short (aren't they all?). The round is actually headspacing onthe extractor. This tilts the case slightly. When fired, there is a moment of gas escape prior to case actually swelling to fill chamber.
Basically, if the gun feeds and cycles and accuracy if there, don't worry.

samsong
08-24-2010, 18:08
Possibility:
The case is too short (aren't they all?). The round is actually headspacing onthe extractor. This tilts the case slightly. When fired, there is a moment of gas escape prior to case actually swelling to fill chamber.
Basically, if the gun feeds and cycles and accuracy if there, don't worry.

Both you and Zombie Steve have gotten me thinking... I have very slightly roll-crimped the rounds to prevent setback from loose bullets (I was actually able to twist them out by hand, like a bad McGuyver episode, lOLz) and I'd bet that the roll crimp may be just enough to let that gas by.

Seeing how it's not causing any problems, I guess I'll just keep doing it like this.

Colorado4Wheel
08-24-2010, 18:49
If your sizing die is not sizing small enough to hold the bullet in place then you need to fix that. The fix is not adding roll crimp on a 9mm. The most you want to do with a roll crimp die in 9mm is remove the bell. You need a better sizer or your brass is too thin.