Case Volume versus Pressure

[leadchucker]

(I'm talking about auto pistol loads here.)

I never have the exact bullet specified in a load listing. I always try to find a load that is as close as possible, as far as weight, jacket type, etc., and work from that, but almost always, the bullet dimensions are different.

I try to calculate the case volume of the load in the listing, and get as close to that as I can with my load, and that has worked well. But there are times when that may not be possible, such as if the needed seating depth would cause my load to exceed the max COL. It has happened.

So I have to increase the seating depth above what the listed load inferred, thereby reducing the case volume. Yes, that will increase pressure, so I reduce the starting powder charge some to compensate for it. I try to reduce the start charge by an equivalent percentage to the reduction in case volume.

Is this charge reduction conservative enough?

 

[brickeyee]

Not close enough depending on what pressure you are operating at.

Pressure is NOT a linear function.

 

[rcmodel]

I try to reduce the start charge by an equivalent percentage to the reduction in case volume.

Don't do that!

That is what the Starting Load is for.

To make up for differences in seating depth, etc.

You don't generally want to reduce a Starting load any further.

rc

 

[sexybeast]

What are you trying to do? Thats the question.
Most everyone I know loads for accuracy. Mass is mass with bullets. Ever weigh a bunch of your bullets. They are different, some as much as 2gr+/- which will affect variations of pressure more than a couple thousanths of bullet set. Look at the difference in max loads for FMJ with titegroup???
YOu have a ten grain bullet difference and the load changes quite a bit .4gr.
This reloading game is about experimentation and lots of it. I never get the velocities show in the reloading tables either and I expect that the pressures being shown aren't correct either but I have no way of knowing.
So, start below max always and see how they do. I see so many "experts" quoting case volume and thousants of an inch OAL that my eyes just glaze over. So many times the OAL listed is completly wrong with the bullet you are using anyway. You have to check the length that the barrel will take.

 

[gamestalker]

Published data has already been tested, and determined to operate safe at necessary seating depths. Trying to reconfigure this is going to risk experiencing other problems, squibs come to mind.

When I think of reduction to minimum published data, I think of all those who have stuck bullets in the barrel. Use the published data as it is written, and providing everything else is correct as should be, you'll never experience serious problems.

GS

 

[leadchucker]

It seems that the common advice on reloading forums is, when in doubt, reduce the charge, or words to that effect. I had the idea that this would apply to starting charges too.

Would you advise against intentionally increasing seating depth over what a published load would be calculated to be?

BTW, there is no "published data" on a large number of bullets, so using some existing published data, and trying to appropriately compensate for the differences is necessary.

 

[sexybeast]

Published data has already been tested, and determined to operate safe at necessary seating depths. Trying to reconfigure this is going to risk experiencing other problems, squibs come to mind.

When I think of reduction to minimum published data, I think of all those who have stuck bullets in the barrel. Use the published data as it is written, and providing everything else is correct as should be, you'll never experience serious problems.

Oh my, here we go again!
If the minimum starting load is 3.5gr and you start with 3.0gr you are not going to have a squib.
You asked about loading for autos so function is also a consideration. I ALWAYS START a bit below starting loads just to see where the threshhold of my gun is. Sometimes it won't be that accurate either, sometimes it is.
Experimentation!!! You are not going to have a squib if you put over 1.5gr of powder in the case. And yes I have tried it.

 

[steve4102]

Quote:
I try to reduce the start charge by an equivalent percentage to the reduction in case volume.


Don't do that!

That is what the Starting Load is for.

To make up for differences in seating depth, etc.

You don't generally want to reduce a Starting load any further.

rc

This is your answer, please do not ignore good advice.

 

[buck460XVR]

Experimentation!!! You are not going to have a squib if you put over 1.5gr of powder in the case. And yes I have tried it.

IMHO, Without knowing the powder/caliber/bullet weight, this is a general statement that would be wise for new re-loaders to ignore.

 

[sexybeast]

A little knowldge is not a dangerous thing. If new reloaders knew exactly what happens then the better for them.
It wouldn't be such a bad thing to try light loads. Bring a squib rod and see how it goes.
Intentionally having a squib may save your butt. You know what it sounds like, you get to see how to clear one, and it may keep you from chambering a round behind a squib and blowing up your gun in the future.
Maybe some of you should just buy factory ammo! Reloading may be too much for you to handle.

 

[LiveLife]

Maybe some of you should just buy factory ammo! Reloading may be too much for you to handle.

Wow. That's like saying "You should just crawl for the rest of your life because you keep falling."

This is the "Handloading & Reloading" category of THR, not "Let's see who is good enough to reload" category.

New reloaders and those interested in reloading come to this section of THR because like all of us, have and will make reloading mistakes and want to learn how to reload safely and effectively.

Our objective is to show that like most everyone will learn to walk even though we fall down many times, we can learn to reload safely even though we are not perfect and make mistakes.

 

[Potatohead]

SexyBeast and BDS Round 3..Ding! Ding! Ding!


You guys are like oil and water..Did one of you steal the others' girl or somethin ?

Sorry, I guess I should butt out.


I had trouble with this question also OP. The way it has been explained to me is that their is a small range of COL that will generally be safe as long as you FOLLOW THE DATA, start with the starting load and work up. And then use the plunk test to make sure it will work in your firearm.

Please correct me if Im wrong somebody, most here are more experienced than I..

I dont feel like Im experienced enough to give much advice, the only reason I chimed in is because I feel I can kind of provide a newbs' view. I think some of these experienced fellas sometimes forget exactly what all of this looks like to a new guy...

 

[243winxb]

Leadchucker, Compare Alliant published data on 45 gap and 45acp. Same bullet/powder. Different primer & case volume. A quick look, seems like less powder in the gap to get the save velocity. Didnt check the SAAMI working pressures to compare?? Like RC said "starting load"

 

[Comrade Mike]

No, what you're thinking will not work.

Under pressure and temperatures ranges that show more ideal behavior

P=(nRT)/V

Pressure is not a linear function wrt volume even under ideal conditions. At 15000 psi and thousands of degrees we deviate from ideal conditions quite a bit and require extensive correction factors analytically derived from experimental data. Don't bet your fingers on basement estimates.

Long story short, stick to the manual. They're published with starting loads for a reason.

 

[TroyUT]

Ok here's my question on this and maybe this is along the lines of what the OP was asking as well. As long as I start with with the published starting load is it alright for me to have a longer OAL than published? My thoughts here were that if I set the bullet out farther the pressure would be lower so it should be safe. Am I right in thinking this? Also whats this "plunk test" I hear so much about?

 

[LiveLife]

As long as I start with with the published starting load is it alright for me to have a longer OAL than published? My thoughts here were that if I set the bullet out farther the pressure would be lower so it should be safe. Am I right in thinking this?

Yes. If you are using OAL/COL longer than published for the same type of bullet, then you should be OK.

Also whats this "plunk test" I hear so much about?

It's the barrel drop test where you take the barrel out of the pistol and drop your dummy test round (no powder/no primer) starting with SAAMI max OAL/COL and decreasing the length until the round falls in freely with a "Plonk/Plunk" and spin without hitting the rifling. Walkalong has a thread that illustrates this well here - http://www.thehighroad.org/showthread.php?t=506678

The barrel drop test will allow you to determine the maximum OAL/COL for the barrel. Next you want to determine the "working OAL/COL" by feeding/chambering the dummy test round from the magazine and releasing the slide without riding it. You may need to incrementally decrease the OAL/COL until the round works reliably. This is the "working OAL/COL" you want to use instead of the published OAL/COL for the powder work up from the start charge.

 

[Comrade Mike]

As a rule of thumb, longer COALs are safer because you're increasing case volume and decreasing pressure. I usually use a COAL a bit longer than the manuals to mitigate any slight variances in my powder charges.

when you go shorter than manuals you have to tread carefully.

 

[1SOW]

When using a different bullet than the data uses:

SEATING DEPTH, for the different bullet is the issue not COAL.
If you know the length of the load data's bullet, you can find the seating depth. Your new bullet (OF THE SAME WEIGHT or very close) can be seated to the same depth and work fine if it passes the plunk test. If the new bullet is .05" shorter than the load data bullet, the published COAL can be shortened .05" and vice-versa if the new bullet is longer.

Other than that, use the starting load.

 

[brickeyee]

At 15000 psi and thousands of degrees we deviate from ideal conditions quite a bit and require extensive correction factors analytically derived from experimental data.

More important, the gasses produced by deflagrating powder are VERY far from 'ideal.'
All sorts of chemical reactions are occurring in the witches brew produced.

 

[James2]

"I never have the exact bullet specified in a load listing. I always try to find a load that is as close as possible, as far as weight, jacket type, etc., and work from that, but almost always, the bullet dimensions are different."

I can only tell you what has worked for me. First let me tell you I have only been reloading for 56 years. For the first 46 years of that I never even had a dial caliper. Also there is no way for me to measure pressure. You can see some tell tale things when it starts to become too high.

OK, here is what I do: Look at several manuals for a bullet of same weight and similar construction. Pick a load and use the start load. Now load a few and go shoot them. You can work up from here.

I have never had a problem doing this. I seat bullets to where they should be seated. I don't need a caliper measurement to see when it is right. There may be some diff in volume from one bullet to the next, but by starting at the Start Load I have never had a problem.

The folks who publish the data give us a start load for a reason. Always start there and work up.