Interior ballistics question

[Digital Dan]

Given:

-the cartridge of your choice rendered magical by virtue of having the ability to increase or decrease capacity.
-All else is equal, including the powder charge

Find:

-a formula or rule of thumb to determine changes in pressure/velocity by virtue of changes to load density.

[mongoose33]

Ummmm....why?

[Artful]

http://www.accurateshooter.com/gear-reviews/test-quickload-review/

http://www.neconos.com/

http://www.jbmballistics.com/ballistics/software/software.shtml

[bhn22]

As Artful suggested, there are computer programs for this sort of thing, based on case volume, bullet seating depth. bullet type, case manufacturer, blood type and shoe size.

[Larry Gibson]

Given:

-the cartridge of your choice rendered magical by virtue of having the ability to increase or decrease capacity.
-All else is equal, including the powder charge

Find:

-a formula or rule of thumb to determine changes in pressure/velocity by virtue of changes to load density.

Wishful thinking and not a new wish as it has been pondered for many years.

If you read the fine print in many reloading manuals you'll find the burning characteristics of many powders are dependent on pressure which, given all else being equal, is dependent to a great degree on load density. If such a magical rule exists there are probably too many exceptions (+/-) to the rule for it to be reliable. The only way to know if the shot in question is within the rule or an exception is to pull the trigger.

Having measured the velocities of thousands of shots since the mid '70s and measured the pressures and velocities of thousands more since '06 I have observed too many anomalies to believe there is such a "rule of thumb". Consider this; if there were such a rule and it was even remotely reliable would it not be in use?

Larry Gibson

[frnkeore]

The program, Quick Load will do all that you ask. You change any or all Perimeters and get a resonable estimate of what might happen but, there are no absolutes, even with tested loads.

Change the primer, primer lot, powder lot, case volume (brand), bullet bearing area & hardness, even the hardness of the brass can get a different result form anyone elses test data.

Frank

[Digital Dan]

Larry, thanks for the feedback on that. Much as I thought, but I'm old enough to know I don't know everything. Thought occurred and it seemed worth the inquiry.

if there were such a rule and it was even remotely reliable would it not be in use?

Probably, but there is much obscure minutiae in the world of internal ballistics of which I am ignorant.


If you read the fine print in many reloading manuals you'll find the burning characteristics of many powders are dependent on pressure which, given all else being equal, is dependent to a great degree on load density.

Indeed. LD is often considered as a function of increasing or decreasing charge weight for a given cartridge. My question sort of backs into the same thing in context of wildcat design where bullet weight/material and velocity goal/objective is known, but case capacity is easily varied by design case OAL. In general practice my experience has it that high LD is part of the precision package for the most part. Trying to define "wiggle room" I guess.

I recall another rule of thumb that is not precise but has some underpinnings of fact wherein at the upper range of practical charge weight and increase of velocity of say 3% brings with it a 9% increase of pressure. V+(%^2) = Pres.+(%^3) or something along that line. It isn't something I'd hang my hat on, but I've seen it repeatedly in print over the years.

Thanks again,

Dan

[303Guy]

Powders do funny things. My favorite is Li'Gun. It produces a higher velocity in a standard hornet than in a k-hornet. Go figure! In some cartridges it is a fast powder while in others it's a slow powder.

BL-C(2) is another curious powder. It's shown as producing higher velocities with lower pressure in some cartridges while behaving like any other in others. Look at it's performance in the 303 Brit and 308 Winchester, two similar volume cartridges.

[leftiye]

Very often in any given cartridge there is a certain powder that simply outshines all others. Does things the others can't. Higher velocities often coupled with lower pressures. I believe this is the inverted view of 303's observation. Often this is hidden in the data and you have to test your gun to see which powders actually will do what the reloading manuals say they will. Then for top all out hunting loads, you must test for accuracy and see if they produce the best groups of the top powders.

[44man]

For me it has always been accuracy first.
I will have no idea at all of velocities until I have the load and decide to run it over the machine. Many never see the chrono. I hate to set the critter up! One revolver I use a dozen boolits out of but can only tell you what one does.
I worked some loads with it and got one gun to 1 SD and my slingshot was better so even now I ignore those figures.
Maybe the worst thing to do is to try and reach printed figures, just too many variables.
Larry has done more then anyone with his testing and I bet he will agree that accuracy counts more.

[rsrocket1]

There is a moderately large difference in pressure and a minor difference in velocity with a bulkier powder than a denser powder when the charges are at 100% or slightly compressed. There is very little difference in either when the charges take up 90% of the case.
Here is a quick and dirty QL run:
powder 1: 1.63 g/cc
powder 1A: 2.2g/cc <- a 25% denser powder with the same other characteristics

4g in 40 S&W with a 180g lead bullet seated to 1.135"COL/0.258" SD
Powder 1: 13,852 psi peak pressure 796 fps out of a 4" barrel
Powder 1A: 13,013 psi, 784 fps

5.42g same two powders
Powder 1: 26,000 psi, 1009 fps
Powder 1A: 23,586 psi, 990 fps

So yes, it can make a difference, but it is highly conditional. Load it to fill up less than 100% of the case volume, the effect drops dramatically. With compressed loads, the pressure difference can be a lot bigger.