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Old 01-04-2013, 11:35 PM
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Default Case volume vs. pressure

I have a mod 19, and reload both 38 and 357. Here's the question...

In any particular reloading guide, For 38, with a particular bullet weight, there is a linear relationship between the grains of powder and the velocity. The same is true for 357.

The difference seems to be between the two. If I interpolate between the two pages, the relationship is not linear.

In 38, an additional .2 grains of powder may give me an additional 50 fps. In 357, an additional .2 grains of powder may give me an additional 50 fps. The question is that the difference in powder loads between the max 38 and the min 357 is not .2 grains per 50 fps.

Am I safe in assuming that this is due to the difference in the length (ie. case volume) between the 38 spl and 357 (assuming bullet seating is at the same place)?
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Old 01-05-2013, 07:17 AM
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Smokeless powder is weird.

Yes , there is a relation between case volume , load density* , and pressure.

*Load density is the proportion of powder to case volume with bullet seated.

BUT , higher pressure does NOT always equal higher velocity.

Powder burn rates is another factor.
Different powders burn at different rates.
Some do not like being compressed.
Some do not burn completely when not slightly compressed.
*Some data sources list load density.
*LD over 100% = compressed load.

So assuming an equal amount of powder with the same bullet is safe in .357 mag cases just because it's safe in .38 Special , or vice-versa , is not a given. It can cause a low powered round , or it can be over pressure.

STICK TO THE DATA PUBLISHED IN MANUALS.

Last edited by mkk41; 01-05-2013 at 07:20 AM.
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Old 01-05-2013, 07:29 AM
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Quote:
Originally Posted by mkk41 View Post
Smokeless powder is weird.

Yes , there is a relation between case volume , load density* , and pressure.

*Load density is the proportion of powder to case volume with bullet seated.

BUT , higher pressure does NOT always equal higher velocity.

Powder burn rates is another factor.
Different powders burn at different rates.
Some do not like being compressed.
Some do not burn completely when not slightly compressed.
*Some data sources list load density.
*LD over 100% = compressed load.

So assuming an equal amount of powder with the same bullet is safe in .357 mag cases just because it's safe in .38 Special , or vice-versa , is not a given. It can cause a low powered round , or it can be over pressure.

STICK TO THE DATA PUBLISHED IN MANUALS.
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Old 01-16-2013, 12:06 AM
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I ask because I see very little load data for +P rounds. I would like to load in this range, and was wondering about the difference between a 'hot' 38 spl and a 'sub' 357.
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Old 01-16-2013, 01:23 AM
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To be both both blunt and brief I have been loading for many years
and own many loading manuals published many years apart and have
no idea how you came to the conclusion than in any given cartridge
there is a direct linear relationship between grains of powder and
velocity. The 38 spl and 357 mag are actually two very different
cartridges that have different optimal working pressure ranges. Some
powders have some application in both but optimal powders in each
will have different burning rates. Smokeless powders are progressive
burning and burning rate changes as pressure increases so a specific
increment of charge increase cannot yield a consistant incremental
increase in pressure and velocity. A hot 38 spl may very likely be at
a lower pressure than a mid range 357 mag load and the two will
contain different powders with different burning rates that are at
different points on their respective progressive curves. There is actually
quite a bit of data in loading manuals for +P 38 spl loads but most of
it is at pressure levels barely above standard. I don't think there is
any thing wrong with a bit of extrapolation in loading for lower pressure
cartridges like the 38 spl and your best friend will be a chronograph.
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Old 01-16-2013, 02:46 AM
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I was about to say a chronograph will be much more useful than 357 Magnum data when trying to develop a .38 Special +P load. If there is no data for a certain bullet or powder in the .38 Special +P it's possible using those bullets or powders is not recommended. BUT, like said above, there is tons of .38 Special +P data in manuals and online.

What are you trying to load and maybe we can help you? Which bullet, bullet weight, brass, primers and powder are you using for that .38 Special +P load? I have several good loads I worked up a few years ago that might fit with what you are doing.
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Old 01-16-2013, 03:53 AM
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Attempting to interpolate or extrapolate any load data is a fools exercise, plain and simple. Doing so is a recipe for an accident.

Bruce
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Old 01-16-2013, 05:37 AM
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Attempting to interpolate or extrapolate any load data is a fools exercise, plain and simple. Doing so is a recipe for an accident.

Bruce
A fools exercise? Nah.

Interpretation and extrapolation of pressure versus volume data has to be quantifiable, predicable and measurable.

A scientists exercise is more like it.

But that I ain't so I won't fool with it.
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Old 01-16-2013, 08:29 AM
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Originally Posted by blujax01 View Post
A fools exercise? Nah.

Interpretation and extrapolation of pressure versus volume data has to be quantifiable, predicable and measurable.

A scientists exercise is more like it.

But that I ain't so I won't fool with it.
Well, I am a no-kidding physicist, and tell you for true the assumptions in the OP are NOT CORRECT. There is a relationship betweeen case volume and pressure ( it is called the expansion ratio), but it is NOT LINEAR. Safe reloads work on a small part of the pressure curve that is flat enough to appear linear, but as you go higher, it becomes exponential. You can stand in the middle of an OK wheat field and swear the earth is flat, but that does not make it true.

In plain English, extrapolation of pressure data IS a fool's exercise. Internal ballistics are not only more complicated than most people imagine, it is more complicated than most people CAN imagine. Actual internal ballistics equations are a system of partial differential equations of the first and second order.

Here's a simplified summary
Internal Ballistics
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Old 01-16-2013, 09:19 AM
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Quote:
Originally Posted by OKFC05 View Post
Well, I am a no-kidding physicist, and tell you for true the assumptions in the OP are NOT CORRECT. There is a relationship betweeen case volume and pressure ( it is called the expansion ratio), but it is NOT LINEAR. Safe reloads work on a small part of the pressure curve that is flat enough to appear linear, but as you go higher, it becomes exponential. You can stand in the middle of an OK wheat field and swear the earth is flat, but that does not make it true.

In plain English, extrapolation of pressure data IS a fool's exercise. Internal ballistics are not only more complicated than most people imagine, it is more complicated than most people CAN imagine. Actual internal ballistics equations are a system of partial differential equations of the first and second order.

Here's a simplified summary
Internal Ballistics
I don't think that can be said loud enough when I read this kind of thread.

This is the same kind of logic that scares me when I read about people testing and looking for pressure signs like a flat primer. Yes, it's a sign but that's a scary sign sometimes.
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Old 01-16-2013, 10:49 AM
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Hey!!

You're misunderstanding me. I agree that it's not linear nor is it a thing the layman should mess with.
It is a measurable, predictable phenominum.
It is a scientific pursuit best left to scientists.
It is only foolish for those without the bona fides.
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Old 01-16-2013, 12:30 PM
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Default Sample MV vs Load and MV vs P vs Seat Depth

I'm sure this is way over the heads of most reloaders and irrelevant to those who simply want safe loads for their guns. For that, please just look up the loads in manuals.

If that's the case, please ignore this post. It is meant only for those who want to get a conceptual understanding (not exact numbers) of the relationship between powder charge, pressure and case volume.

Here are some sample loads I ran on Quickload a while ago when I was also thinking about what the OP was talking about .
Typical disclaimer : THIS IS ONLY CALCULATED DATA AND NOT MEANT TO BE ACCURATE LOAD DATA. DON'T EXPECT THIS DATA TO BE SAFE FOR YOU.

That being said, Quickload has been very reliable for me when predicting safe middle of the road loads especially when comparing one powder to another and one bullet weight to another. It does diverge when predicting muzzle velocities in very short barrels (2"). It tends to slightly overestimate 45ACP velocities (compared to what I've chrony'ed) and it underestimates handgun load velocities when you start loading hot (for instance 38 sp with a load of 5g Unique under a 158g bullet). I never use a load greater than 75% predicted SAAMI max.

Here is a plot of MV, Pressure and Energy when a 45ACP 230g LRN is loaded with Unique:

The yellow markers are what I consider the max safe loads.

You can see that in this small section, velocity does increase almost linearly with powder charge, but pressure (your safety margin) increases exponentially. That means when reaching for more velocity, you reach the pressure danger level a lot quicker as you increase the powder charge.

The next graph is a plot using the same bullet, same charge, but seating the bullet deeper each time. I think it was with a 5.6g charge:


You can see that MV does go up with increasing seat depth, but pressure goes up much faster. That's why putting 4.5g Unique under a 125g bullet in a 9mm case comes out faster with a lot more pressure than the exact same bullet with the same charge in a cavernous 38 special case.

None of these plots should be taken as absolute, but you can see the relative relationship of charge variation vs case volume variation on pressure and MV.
Hope this satisfies a little of your curiosity.

Last edited by rsrocket1; 01-16-2013 at 03:14 PM. Reason: preamble
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Old 01-16-2013, 01:19 PM
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If you're new to reloading stick to the manuals. A chronograph is a handy tool. You can turn pretty much anything into a complex math equation, loading ammo isnt that difficult. Charge weight to pressure isnt linear.
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Old 01-16-2013, 05:59 PM
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Black powder - natural - good magic!

Smokeless powder - evil - bad magic !
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