Powder density in Pistol Cartridges

[Oyeboten]

I am feeling confused about the consideration of Airspace or relationship of Powder Volume and Airspace in Pistol Cartridges...and how these can affect pressures realized.


I am told that the same volume of Powder under a same weight Bullet can have very different pressures if more or less airspace is present...or, can generate overpressures, if too too little airspace is present, suggesting then, that for any given powder, there will be an optimum airspace?


This began in a conversation about .455 Eley in a Colt New Service Revolver, in comparison to .45 LC, where the volume of the Cases are different...and, I thought it would be alright in this Case, for this Revolver, to use more or different Powder in the .455 Eley Case, to approximate the .45 LC loadings.


I was told this might be a bad idea, since the .455 case is so short, the then too little of airspace could cause an overpressure condition.


How is one to understand this?

 

[DaveBeal]

It's due to a principle of physics called the Ideal (or Perfect) Gas Law, which says that for a fixed quantity of gas, volume and pressure are inversely proportional. So, for instance, if you squeeze some gas down to half its original volume, its pressure will double. If you allow it twice its original volume, its pressure will be halved.

Assuming that a given amount of powder always creates the same amount of propellent gas, this gas will initially have twice the pressure if its combustion is confined to half the space. This is complicated by the fact that the bullet begins to move, allowing additional space, before the powder is completely consumed, but the principle is still valid.

That's why, even when using a specific powder charge in a specific case, bullet seating depth (usually measured as cartridge overall length) is an important safety factor in reloading. Seating the bullet too deep can cause overpressure.

 

[Oyeboten]

Hi DaveBeal,


Very nice explaination...thank you..!


Of course, with Black Powder, one does one's best not only to have no empty space in the Cartridge, but, one compresses the Powder definitely in seating the Bullet, thus reducing even such little Airspace as would otherwise be among the Powder granules.


'777' one does not compress, but, one loads so as to have 'zero' open space or Air Space.


When using Bullseye, Unique or other Powders also, when for .38 Special and HBWC or Flat Base WC, one reduces the erstwhile Air Spcae by intentionally seating the 148 or 158 Grn Bullet to be flush with the Case Mouth...even though same Powders and same Charges would be used for normally seated 158 Grn RNL or semi-wad-Cutter Bullets.

In SD or Longer Range Cartridges in .38 Special, using 158 Grain full Wadcutters, one does the same, seating the Bullet to be flush with the Casemouth, but with a larger Powder Charge.


This has always been said to be more 'effecient' in .38 Special, yes? Seating the Bullet deeply and or flush with the Case Mouth when using Smokeless Powders of low volume and high pressure yield.



So...

How do I reconcile those occasions where one is alright to either have smaller empty space from Bullet Seating depth, or a smaller empty space from more powder AND a deep seated Bullet, and, those occasions where, appearantly, one is not alright?

 

[918v]

Generally speaking, the deeper the bullet, the higher the pressure given the same powder charge. 38 Special wadcutter target ammo is not loaded to maximum pressures, while 158gr SWCHP is. If you were to load both to maximum pressure, the deeper seated wadcutter would require less powder.

 

[jr_roosa]

It's not the "air space" that matters. It's the ratio of powder to internal case volume. Gunpowder doesn't need air to burn.

.38 special is a weird example because the case volume is huge compared to how much somkeless powder you use, and because the most common target loading (148 gr HBWC) is loaded very weakly compared to max pressure. It's because it's a carryover from the black powder era and once needed all of that case volume to hold a much larger charge of black powder to reach the same max pressure.

So yes, in a .38 special it's OK to seat flush and then up the powder charge to make a SD load without blowing up the case, but it's only because you're not starting from a maximum pressure load and there's a lot of leeway.

In high pressure/low volume cases like 9mm, 40SW, etc, you can get huge pressures from seating the bullet too deeply, even if it's just by a little bit.

-J.

 

[Oyeboten]

Thanks very much you guys...this has helped me get a handle on this.

When I was re-loading a lot in the 1980s, I just followed carefully the basic loading data for the Target Rounds I was making, and I never strayed...never thought about theory or other dimensions of it.


A few months ago, starting to re-load again, I limited myself to Black Powder and '777' only, both of which fill the case in their way...intending to stay on those a while before moving on to Smokeless.


So...since I am on a learning curve now, rather than merely doing things by rote, this is a valuable topic for me.


So, in a Nut-Shell then, any specific volume of a given 'smokeless' powder will produce such pressure as it does, and this pressure will be spread out over the volume of space it has to do it in according to the Powder/Propellent's rate of conflagration under the conditions, thus giving variable PSI depending on these conditions...so less space to do it in = highest pressure....more space to do it in = same potential for pressure realized, but pressure is lower for being spread out into a larger volume, and thus less PSI.

 

[Jesse Heywood]

so less space to do it in = highest pressure....more space to do it in = same potential for pressure realized, but pressure is lower for being spread out into a larger volume, and thus less PSI.

For the most part, yes. Other variables include temperature, barometric pressure, humidity, powder position, quirks from powder batches and primer runs, cartridge dimensions, bullet weights and sizes, crimping, chamber size, barrel length, and on and on and on.

That's why most of us here stick to the books, and find multiple sources for the load we are looking at. Helps keep the kaboom factor down, which relates to the volume of common sense used.

 

[1SOW]

If you double the length of the barrel of your Daisy Red Rider, the BB is going to move much slower, affected by coating and weight of BB, barometric pressure/air density and humidity.

Now if you double your spring weight you might blow your rubber seals....

 

[Oyeboten]

Thanks Jesse Heywood...makes sense.


Are there any published Tables which state how much Gas a given Powder makes? Or, rather, how much PSI if confined with no Space?


Seems like it would be a nice thing to have, if there was...


I understand that a Powder's Burn rate, Burn rate at what pressure...resistence of the Bullet to initial movement and then to Forcing Cone or Rifling, all must play a part in the duration a Chamber obliges an initial pressure.

Even to where some overcharges can blow a Chamber/Receiver before the Bullet has even moved...

Brooding...