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Old June 30, 2020, 12:02 PM   #1
mpmo
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Join Date: June 29, 2020
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Brass shrinking, adjust OAL's? Anything else?

Hey y'all.

New reloader here. i just collected my brass from my first test rounds and noticed that it is .001" shorter than it was when I started. Starline 9mm, brand new to start. It is now .749 down from .750. This is measured after using the lee resize/de-prime die.

So if my brass is going to continue to shrink, do I need to adjust OAL in order to keep pressures consistent? Are there any other adjustments that might need to be made?

What is all this about trimming when the brass actually shrinks? Will it grow at some point?
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Old June 30, 2020, 12:21 PM   #2
nhyrum
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Join Date: March 9, 2016
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There's no need to adjust oal every time. Yes, rimless cartridges shrink. But it usually stabilizes quick. Kind of like but copper build up in rifles. From a new clean bore, copper get deposited relatively quickly for the first bit, then shallows out.

So no, I'm not aware of anyone who adjusts oal with shrinking brass. I usually pick an oal I'm shooting for, load everything that length then load to either the power factor I want, or with rifle, till I find the accuracy and Chrono numbers I want

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Old June 30, 2020, 05:21 PM   #3
Unclenick
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No need to adust anything but your crimp die as it gets shorter. A lot of pistols have long chambers, and even though they are nominally headspacing on the case mouth, many actually are stopped by the extractor hook before they go that far forward and are actually headspacing on the extractor.

You can check for that easily enough. Make up a dummy round and push it into the chamber with your pinky and then put a dowel in from the muzzle and mark it next to the muzzle. Repeat with the dummy chambered in the gun and the muzzle up so the dummy rests on the breech face in the slide. If the difference between the two marks is greater than the distance from the breech face to the front inside of the extractor hook minus the rim thickness, then you are headspacing on the extractor hook anyway and aren't depending on the case mouth at all. You may need to replace your extractor from time to time, but otherwise, this is pretty common. One of the board's gunsmith members estimated 70% of the 1911's he'd seen come through his shop were headspacing on their extractor hooks.

I once took a large set of 45 Auto brass through 50 reloadings of target level loads. They were 0.025" shorter at the end of the experiment, having lost an average of half a thousandth at every load cycle. This is due to the chamber taper which is filled by the brass at every firing, such that a tiny portion of the brass is permanently flowed to the rear by the resizing die during each load cycle. By contrast, straight-wall revolver cartridges seem to remain pretty stable.

If you start loading a cartridge whose peak pressure is much above about 30,000 psi, then you can start to see cartridge case stretch at the pressure ring that forms, and the need for trimming starts to occur.
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