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Old August 17, 2001, 07:11 PM   #1
slickpuppy
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Case length versus pressure

Hi guys,
I finally started wading through some of my once fired Winchester brass in 9mm. I sized, decapped, and cleaned it and curiosity kinda got to me, plus I was stuck inside because of rain.

I started measuring the length of these cases and had variances in length from .750 to .743. I wasn't really surprised there since I shoot numerous 9mm pistols and would expect that.

Common sense tells me to adjust the loaded cartridge OAL to maintain a consistent seating depth with the different brass lengths.

My question to you guys is this: Keeping the loaded cartridge OAL constant, what kind of pressure differences have you seen with different lengths of sized brass with this kind of variance?


Thanks
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Old August 17, 2001, 10:35 PM   #2
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If you keep the OAL length the same you won't be changing the depth that the bullet is seated in the case regardless of the case length. The crimp will be different on shorter brass than longer, but not the case capacity if you use the same OAL with varying lengths brass.

What will effect pressure is the ammount of space inside the case, which is the distance from the base of the brass to the base of the bullet, that's what determines case volumn, not the distance from the tip of the bullet to the top of the case.

Keep the OAL length the same, I generally load to nearly the max length my magazines can take regardless of the length of the brass.
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Old August 17, 2001, 10:39 PM   #3
Mal H
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Good question slickpuppy, I'll give it a shot.

I believe that if you use a single OAL you shouldn't see a noticeable pressure difference above the normal range even though some cases may be significantly longer than others. The reason is that the difference in case dimension will all be taken up in the bullet seating area and not in the powder cavity, that should be a constant with a constant OAL. The small amount of "purchase" difference on the bullets is not nearly as important for pressure buildup as it would be with variations in the powder cavity.

In other words, if you vary the OAL to compensate for the difference in case lengths, you will vary the pressure more than if you kept a constant OAL with varying length cases, varying within a small range of course.

Clear? Or mud?

If someone else is well versed in interior ballistics, I also would like to hear the "official" answer to this.
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Old August 17, 2001, 10:40 PM   #4
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What are the odds, John? Slickpuppy's post sat here for 3 hours and then we both responded at the same time.
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Old August 17, 2001, 10:56 PM   #5
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It's karma, the timing just needed to be perfect for that question I guess, we just had to wait for the stars to align and the spirits to be willing before the answers would come to the proper people.
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Old August 17, 2001, 11:08 PM   #6
slickpuppy
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The crimp will be different on shorter brass

JohnK,
How will my crimp be different? I don't see that. Logic tells me that a case .743 inches in length versus a case .750 inches in length and loaded to the same OAL with a seated bullet is definitely going to have less bullet in the interior of the case. To me, that means a definite pressure differential between the two.

I'm not trying to be argumentative here. I simply don't understand your crimp difference explanation.

Mal,
Considering the interior ballistics of the case, you raise some interesting questions. With a case variance of .007 in a high pressure cartridge like the 9mm I would think there would be a very noticeable pressure difference.

Perhaps its time to do some testing.

Thanks for the responses gentlemen. I look forward to more.
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Old August 17, 2001, 11:15 PM   #7
Mal H
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slickpuppy, I'm not positive, but I think you missed the point both John and I made. You won't have a .007" case volumn difference if you use a constant OAL. The distance from the base of the bullet to the inside of the case head will be the same for different length cases. It is the case volumn (the volumn below the bullet base) that makes the biggest difference in pressure variations.

The instant the powder ignites the pressure rises extemely fast, the case expands to tightly fill the chamber and the amount of bullet inside the case makes little difference. However, if you set the bullet deeper than the prescribed OAL, then the pressure rises even faster and higher due to the decreased volumn available for the gases to expand into. It's a chemical thing and a phenomenon peculiar to nitrocellulose and other NG derivatives.
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Old August 17, 2001, 11:26 PM   #8
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Ok, I'll tackle the crimp difference first. You screw your crimp die down to get X crimp on a case of Y length, correct? So if you put a longer case in it will get more crimp since it's going further into the crimp die, the same as if you'd screwed the die further down for shorter brass. Essentially if you set the crimp die for the short brass you will have more crimp on the longer ones. The differnece in length you have is only .007 so the difference in crimp isn't going to be dramatic, just something that will be slightly different.

I think you're focusing to much on the case length when thinking about the pressure of the round. What causes the pressure? It's powder burning inside the case, filling the available interior case volume.

A 9mm 115gr FMJ bullet is .551" in length. If you seat the bullet so the cartridge OAL (length from the bullet tip to the outside base of the bullet) of 1.12" you'll have .569" from the base of the bullet to the base of the case, regardless of the case length. Using longer brass just means that more of the bullet will be inside the case but the interior volume will remain the same.
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Old August 17, 2001, 11:40 PM   #9
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I think a visual aid will help. The base of bullet line isn't to scale, but it's roughly correct. You can see if you make the case longer or shorter but don't change the OAL the interior capacity of the round doesn't change either.

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Old August 17, 2001, 11:47 PM   #10
slickpuppy
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John, I think I understand what you are saying

The volume of the case will not change with differences in case length because what happens is that the diameter of the case will increase when the bullet is crimped in the shorter case. That has to happen. The brass has to go somewhere.

Therefore, the true volume of the case is not less. It is the same as the longer case. It simply means that the case dimensions have been changed to keep volume constant. The brass has to go somewhere and that somewhere is in case diameter!


Now, I think I understand what you are saying.

OK, so that is settled. NOW, what are the powder burn characteristics of this case?
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Old August 17, 2001, 11:49 PM   #11
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Uncle!



Next up ....
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Old August 18, 2001, 12:02 AM   #12
slickpuppy
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John, You posted the drawing while I was writing

I see what you are saying now. The area between the base of the bullet to the end of the cartridge case may be different, but, that should have minimal affect on the pressure of the round.

Another interesting question arises to me during this discussion.

Given different lengths between the bullet base and the end of the brass, what is the effect of crimp on a longer or shorter length of this area? Perhaps I am looking at some tests?
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Old August 18, 2001, 04:27 AM   #13
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A difference of 743 to 75 is within tolerances. You'll notice no differences in bullet performance or handle-pull on the loader. The crimp may be a little deeper on the longer cases (depending on the type of crimp you use), but it shouldn't affect performance, unless you're loading match grade.

Although I usually don't crimp, that's why I always trim my casings. I see it as just one of the steps of loading, instead of something I only do occasionally.
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