I may be a little wrong in the wording here but the case mouth ( in auto pistols ) is what stops the cartridge in the barrel.
Not the shape of the bullet.
This is how all my automatic pistols work anyway. 9mm 40S&W and 45auto.
So case length and taper crimp have something to do with this.
If you look inside your barrel where the plunking takes place and the cartrdige rests, there is a very thin ring inside the barrel that the cartridge stops at and the bullet starts.
I suppose it might be possible to taper crimp a cartridge to the point where it would not rest there and go too far forward and plunk below the the diagrams shown. A short case would do the same.
There is no need to trim your 9mm brass. It is good to chamfer and deburr them.
If it is too short tho, throw it away. It will go too deep.
If you seat your bullet too deep it will increase the pressure in the case and if you seat the bullet too long it may not feed correctly while loading cycling and firing.
BOTH CAN CAUSE INJURY and/or damage the firearm.
Different loadings will sometimes require die adjustments because of the size, shape and weight of the bullets.
But if you stick with a certain favorite combination that your pistol likes and it functions with your proper loads, you can adjust and lock your dies in place and they will make the same size loads for your pistol every time without having to mess with them using the same bullets.
The plunk test is not the only factor in making a cartridge safe and functional in your pistol.
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