The case of the incredible SHRINKING brass!!

[Dusty Miller]

I trimmed a bunch of brass to .888" (.45 ACP), loaded it up, took it to the range and shot it all.  Now when I resize that same brass about 1/3 of it is coming out between .884" and .886".  WHUT'S GO'N ON HERE?!!  I thought brass was supposed to STRETCH when it was shot.

[hogship]

DM....

I don't really have the answer as to why 45acp gets smaller, but I've noticed the same thing.

I have a batch of Winchester brass that was originally purchased as new unprimed cases, and have never been trimmed. After an initial FL size, they were within .891"-.895" in length. Now, after 5x reloads, they are .887"-.893" in length.

I don't bother with trimming 45acp, or any other straight wall handgun cartridge cases for that matter, because it doesn't yield any useable accuracy or function benefits....That is, as long as they aren't over length and are pretty much consistant in length......but I always give bottleneck rifle cases a trim prior to the first reload.

I do have a question for you, though. Why did you choose .888" as a trim-to-length setting? Was this for overall consistancy? As with all cartridges, unless you trim on each reload, you're cases will never maintain constant lengths, as trimmed.

hog

[calvon]

When I first began loading for the 9mm Luger pistol cartridge I worried and wondered about brass growth. To find out, I started with 20 pieces of new Winchester brass, numbered them 1 to 20, and measured the length of each with an electronic micrometer that will read direct to three decimal places in metric. That third decimal place is one thousandth of a millimeter, or one micron. I loaded and fired them and measured the length after firing. This was followed by a full length resize in RCBS carbide dies and then another measurement. This was repeated for ten firings. All measurements in millimeters. Here is a summary of the results:

New: Mean Length: 19.009 mm. Std. Dev: 0.015 mm

After firing Mean L: 18.854 mm. Std. Dev: 0.020 mm

After resize Mean L: 19.013 mm. Std. Dev: 0.021 mm

You will notice that firing them caused them to shrink and sizing them caused them to grow. The end result, after ten firings, was that the brass grew by four microns. That's less than the thickness of a human hair. I decided that from now on I won't worry about brass growth but will, on a spot check basis, check length once in awhile. This ammo was fired in a P89 Ruger.

[PaulS]

The reason your brass gets shorter is that it expands diametrically. As the pressure expands the case outward it shortens it's length. The pressure isn't high enough to cause brass flow so the case just trasfers its length to its diameter.

PaulS

[Duffy]

And to add to what Paul said you usually can't  resize all the way to the rim so that little section stays large and the brass is shortened. Mike them right after shooting and their shorter yet.  Don't sweat it.

[hogship]

The reason your brass gets shorter is that it expands diametrically. As the pressure expands the case outward it shortens it's length. The pressure isn't high enough to cause brass flow so the case just trasfers its length to its diameter.

PaulS

PaulS......

True, you are exactly right when considering the expanding brass in the chamber as it is fired.....but you are not taking into account that the diameter of the brass is returned to the same diameter after FL sizing.

Why, after FL resizing, using the same dies, does 45acp cases steadily grow shorter?

It would seem that if the case is preped using the same FL resizing die, and without trimming, it wouldn't be possible for the length to become shorter....only longer is possible. The brass that made the case longer had to go somewhere. That is, unless the brass flows into the case base, or the walls become thicker. The reason why the case becomes shorter after FL sizing, isn't readily apparent.

Except for the 2nd reloading, here's the records I logged for the particular batch of Win 45acp brass I mentioned above:

1) .891-.895

2) (no record kept)

3) .888-.894

4) .888-.893

5) .887-.893

As you can see, there seems to be a steady reduction in length throughout the five reloadings of this brass. All were fired in the same 1911 pistol, so that isn't a factor.

Also, we know that, as brass is used it becomes harder, resulting in less resiliency.....because of that, the case would require addittional sizing because it doesn't contract as much as a new case after expanding in the chamber. The result in additional resizing, is lengthening.....but, we know that isn't the case for the current subject.

hog

[Donna]

Hog-

Maybe it’s the same thing as saying wool-clothing shrinks. This can’t be true otherwise each time it rains the sheep would get smaller and smaller. LOL  :-D

Donna

[hogship]

Not my sheep, Donna....

I spray mine with 3M!

The only problem with 3M water repellent is my sheep now float. :lol:

hog