Input Please: Starting Charge For Short 9mm COL

[rsrocket1]

A 5% reduction in volume will raise the max pressure in the load from 35,000 psi to 41,400. Will that blow up the gun? Highly unlikely. But a steady diet of ammo at that pressure isn't good either. On top of that, add another few % because of some bullets that might get set back due to variances in loading procedure. Are you willing to say if 41,500 isn't going to be bad, then maybe 44,000 isn't going to be nuclear either. At what point do you say "you'd better back off"?

 

[otisrush]

IMHO the thing to take away from this discussion is that pressure and volume do not have a linear relationship. So reducing volume a few % does not necessarily mean pressure will go up a few %. It could go up A LOT. It may not......but it could.....depending on the deets such as specific powder, specific case, etc. etc. etc.

Thanks.

OR

 

[Rule3]

IMHO the thing to take away from this discussion is that pressure and volume do not have a linear relationship. So reducing volume a few % does not necessarily mean pressure will go up a few %. It could go up A LOT. It may not......but it could.....depending on the deets such as specific powder, specific case, etc. etc. etc.

Thanks.

OR

Correct, and the point I am trying to make is that seating depth is going to matter more with fast powders than with slow ones, especially in a small high pressure round like a 9mm, It will not matter as much in a low pressure 45 ACP which does better with fast powders,

That said you will have to adjust your seating depth based on your bullet and barrel, so it is not written in stone, If you use the exact same bullet as in the manual than you can just use that data but that does not happen that often.

Going back to your original post, you should be fine if you use the start load just be sure your powder measure is weighed exactly.

 

[otisrush]

I just returned from the range so I thought I'd give a report - just to close this out and leave a self-documenting question and result for internet eternity.

First, I re-setup my dies and I was able to go a little longer and have them plunk successfully. Final length was 1.110. Just a couple of extra thousandths - but I thought I'd take what I could get.

I loaded up 5 rounds each of 4.0gr, 4.1gr and 4.2gr. All of them cycled the action flawlessly. None of them exerted any signs of excess pressure. The 4.2gr showed the greatest promise accuracy wise. But only firing 4 rounds of each load (the first round of each load I was watching the eject so I could trace down that specific fired case to inspect it) isn't much of a test. But my plan is to load up more of the 4.2gr and see how they do.

Thanks for all the input!

OR

 

[Rule3]

"just to close this out and leave a self-documenting question and result for internet eternity."

Al Gore thanks you!

 

[fredj338]

So the impact on volume by a change in the depth of bullet seating is linear (height).

You argue with this ?!?!

Uh yeah & explained why. It may be linear with one powder & bullets, & not all powders. Not understanding that is a reloading disaster just waiting to happen.

 

[fredj338]

A 5% reduction in volume will raise the max pressure in the load from 35,000 psi to 41,400. Will that blow up the gun? Highly unlikely. But a steady diet of ammo at that pressure isn't good either. On top of that, add another few % because of some bullets that might get set back due to variances in loading procedure. Are you willing to say if 41,500 isn't going to be bad, then maybe 44,000 isn't going to be nuclear either. At what point do you say "you'd better back off"?

Blow up a gun, not a well made one. Blow out a case head, maybe. Anytime you are far over pressure, then you risk a bad case blowing out. Again, no way to determine if a 5% volume reduction does this across the broad spectrum of powders. Uber fast, fast, medium, medium slow, slow, too large a pressure swing to randomly assign a number or math equation. Even quick loads is an educated guess. Without pressure testing its all a guess.

 

[otisrush]

Uh yeah & explained why. It may be linear with one powder & bullets, & not all powders. Not understanding that is a reloading disaster just waiting to happen.

I want to parrot back things I think I know (and their relationships) to be sure I get what you're saying:

* There is no question there is a linear relationship between seating depth and internal volume of the case. This situation is completely independent of the particular powder being used. The internal case volume is the internal case volume - whether it has a little or a lot of powder in it. I agree with Pi*R^2*H for calculating volume. (I know the internal of a case isn't a perfect cylinder. But I would argue that there is a linear relationship between seating depth and resulting internal volume that the powder can occupy in and burn in.)
* What pressure is generated as a result of that volume is not only the most important consideration, but also the most difficult to predict. The pressure that is generated is a function of the actual powder used (Does it burn fast or slow? Is it compressed or not?), the bullet weight used, and also a variety of other things that are even tougher to deal with such as actual bullet diameter relative to that particular gun's barrel diameter, whether the bullet is touching the lands or not, etc. All of these variables come together to produce some amount of pressure that, we all hope, is below max.

So this is why I go back to summarizing it as:
* The relationship between bullet seating depth and internal case volume (space powder occupies) is linear.
* The relationship between internal volume powder can occupy and resulting chamber pressure is non-linear.

Am I representing the physics correctly?

OR

 

[BC38]

I want to parrot back things I think I know (and their relationships) to be sure I get what you're saying:

* There is no question there is a linear relationship between seating depth and internal volume of the case. This situation is completely independent of the particular powder being used. The internal case volume is the internal case volume - whether it has a little or a lot of powder in it. I agree with Pi*R^2*H for calculating volume. (I know the internal of a case isn't a perfect cylinder. But I would argue that there is a linear relationship between seating depth and resulting internal volume that the powder can occupy in and burn in.)
* What pressure is generated as a result of that volume is not only the most important consideration, but also the most difficult to predict. The pressure that is generated is a function of the actual powder used (Does it burn fast or slow? Is it compressed or not?), the bullet weight used, and also a variety of other things that are even tougher to deal with such as actual bullet diameter relative to that particular gun's barrel diameter, whether the bullet is touching the lands or not, etc. All of these variables come together to produce some amount of pressure that, we all hope, is below max.

So this is why I go back to summarizing it as:
* The relationship between bullet seating depth and internal case volume (space powder occupies) is linear.
* The relationship between internal volume powder can occupy and resulting chamber pressure is non-linear.

Am I representing the physics correctly?

That is pretty much what I got from it all.

The one bit I can add is that I saw a COAL vs pressure chart published by the manufacturer for Ramshot Zip powder and it showed that a 0.06" reduction in COAL (around 11% IIRC) caused a 17% increase in pressure. Obvioulsy that isn't in any way linear.

 

[Gamecock]

If you loaded 8.7% more powder than the manufacturer's published data maximum load, would you expect to be over SAAMI pressure?

Alliant published loading data says 4.6 gr Bullseye is max for above caliber/bullet/powder data. This test was obviously designed to scare, not to illuminate. It is quite likely that had they used 4.6 BE instead of 5.0 BE, the load would have been within SAAMI pressure guidelines. It is impossible to determine how much of the over pressure is due to seating depth and how much is due to exceeding published max.

Frankly, I'm surprised that the pressure reported wasn't HIGHER.