What causes higher pressure with shorter OAL.

[neckbone]

This was something I didn't realize in rifle reloading. Why is it true with pistol?

[racerba]

Because shorter OAL means the bullet sits deeper into the casing which means lesser space for the same volume of gas being expanded = more pressure.

[GuildSF4]

Because shorter OAL means the bullet sits deeper into the casing which means lesser space for the same volume of gas being expanded = more pressure.

This ...

It is also true in rifle cases, the effect is lower per mil than in pistol cartridges as a mil is a much lower percent of volume in a rifle cartridge than a pistol cartridge. It can cause damage in rifles also though.

[jlamphere]

There is also more surface area of the bullet inside the case. This creates increased resistance and more time and energy are required to get the bullet into motion. The additional time allows more pressure to build behind the bullet before it starts moving.

[gng4life]

Physics...

[Selby213]

So I know that there are max OAL listed in manual's are there acceptable ranges, ie if you are within 5-10% of Max OAL then should be good to go?

[Too_Slow]

If a load listing in a book says X gr. (bullet type) with X gr. (powder) @ OAL , the OAL shown is the MINIMUM OAL not the max. If you are going to make the OAL shorter you have to decrees the powder charge.

Brian

[njl]

Physics...

And chemistry. At least I think it was chemistry where PV=nRT was forever burned into my memory. With nRT remaining constant, reducing V will increase P.

[sigsauerfan]

a bullet seated deeper in the cartridge means more or less more powder. as you decrease free space in the case,it's like adding more powder. on top of that a higher percentage of the bullet surface is tightened by the crimping requiring more pressure to launch the projectile. should re-evaluate the powder charge for each .10 added inside the case,specially if you are playing around and beyond 80% of the max load .

[Red Ryder]

Physics...

And chemistry. At least I think it was chemistry where PV=nRT was forever burned into my memory. With nRT remaining constant, reducing V will increase P.

Yes! But it is not a proportional relationship in reloading.

[twistrate]

Yes, that is correct it is not necessarily a linear relationship a lot of the times. One additional factor is that the initial increased pressure from the deceased volume due to OAL actually affects and modifies the burn rate and characteristics of the powder for the rest of the burn. This "feedback loop" can cause very sudden changes and spikes in charge behavior. This can be seen with slower burning powders like HS-6 and Autocomp. It's one of the reasons people report more complete combustion (less dirty) when they load max loads or major loads with them.

[Smitty79]

Physics...

And chemistry. At least I think it was chemistry where PV=nRT was forever burned into my memory. With nRT remaining constant, reducing V will increase P.

The equation is physics. We just let chemists use it. All science is physics or stamp collecting.

Edited September 14, 2013 by Smitty79

[njl]

Physics...

And chemistry. At least I think it was chemistry where PV=nRT was forever burned into my memory. With nRT remaining constant, reducing V will increase P.

The equation is physics. We just let chemists use it. All science is physics or stamp collecting.

Well, if you want to quibble about it, we could just say it's all math. Try doing some physics without calculus.

As mentioned, the pressure / volume thing would be proportional but pressure tends to alter the burn rate of powders, and as this is happening, the bullet starts to move, increasing volume. Peak pressure will depend on the amount and burn rate of the powder and the movement of the bullet...making the PV equation (solving for peak pressure) quite a bit more complicated than nRT.

[Sarge]

Physics...

And chemistry. At least I think it was chemistry where PV=nRT was forever burned into my memory. With nRT remaining constant, reducing V will increase P.

The equation is physics. We just let chemists use it. All science is physics or stamp collecting.

Well, if you want to quibble about it, we could just say it's all math. Try doing some physics without calculus.

As mentioned, the pressure / volume thing would be proportional but pressure tends to alter the burn rate of powders, and as this is happening, the bullet starts to move, increasing volume. Peak pressure will depend on the amount and burn rate of the powder and the movement of the bullet...making the PV equation (solving for peak pressure) quite a bit more complicated than nRT.

Good lord!! I didn't take any of those courses but I know if you throw an M80 in a 55 gal drum there is less pressure than if you throw it in a gallon paint can.

And please don't respond with a formula that says it depends on the brand of paint.

[Youngeyes]

Physics...

And chemistry. At least I think it was chemistry where PV=nRT was forever burned into my memory. With nRT remaining constant, reducing V will increase P.

The equation is physics. We just let chemists use it. All science is physics or stamp collecting.

Well, if you want to quibble about it, we could just say it's all math. Try doing some physics without calculus.

As mentioned, the pressure / volume thing would be proportional but pressure tends to alter the burn rate of powders, and as this is happening, the bullet starts to move, increasing volume. Peak pressure will depend on the amount and burn rate of the powder and the movement of the bullet...making the PV equation (solving for peak pressure) quite a bit more complicated than nRT.

Good lord!! I didn't take any of those courses but I know if you throw an M80 in a 55 gal drum there is less pressure than if you throw it in a gallon paint can.

And please don't respond with a formula that says it depends on the brand of paint.

Yabbut if it's done in the woods and no one is around, does it make a sound?

[GrumpyOne]

Physics...

And chemistry. At least I think it was chemistry where PV=nRT was forever burned into my memory. With nRT remaining constant, reducing V will increase P.

The equation is physics. We just let chemists use it. All science is physics or stamp collecting.

Well, if you want to quibble about it, we could just say it's all math. Try doing some physics without calculus.

As mentioned, the pressure / volume thing would be proportional but pressure tends to alter the burn rate of powders, and as this is happening, the bullet starts to move, increasing volume. Peak pressure will depend on the amount and burn rate of the powder and the movement of the bullet...making the PV equation (solving for peak pressure) quite a bit more complicated than nRT.

Good lord!! I didn't take any of those courses but I know if you throw an M80 in a 55 gal drum there is less pressure than if you throw it in a gallon paint can.

And please don't respond with a formula that says it depends on the brand of paint.

Wouldn't oil based paint make a bigger boom?

[Sarge]

Friends.........

[MarkCO]

Actually, PV=nRT is the ideal gas law, first described by Clapeyron, an engineer who is considered to be one of the founders of Thermodynamics. The Chemists and Physicists realized the brilliance and have been attempting to claim it as their own for the last 180 years.

There are 7 physical laws, physics has three, Thermodynamics has four.

When we solve combustion problems, we realize we are not working with a point in time nor an ideal gas, so we do not use PV=nRT. We will most often use a derivative of the first law for this type of analysis, usually starting with a quasi-static assumption. Anaylsis of friction, in the form of deltaQ in the first law reveals there is insignificant contribution to work, thus when the energy balance is figured, it has no relevant contribution.

If you want to read about it, find Zucker's "Fundamentals of Gas Dynamics". He does a pretty good job, but it is a graduate level book that assumes you have taken a few semesters of undergraduate Thremodynamics.

For the lay person, read Sarge's response in post #14...he gets it.

[Sarge]

For the lay person, read Sarge's response in post #14...he gets it.

Don't know whether to say thanks or F off!

[MarkCO]

Common sense is often easier, and sometimes better than the math (which is really just an approximation). No dig on you at all Sarge, much the contrary.

[Sarge]

I know. Just mess in' with ya'

[thegunnerd]

Physics...

And chemistry. At least I think it was chemistry where PV=nRT was forever burned into my memory. With nRT remaining constant, reducing V will increase P.

The equation is physics. We just let chemists use it. All science is physics or stamp collecting.

Awesome ! I was just saying a similar thing about physics earlier

[ChuckS]

Good lord!! I didn't take any of those courses but I know if you throw an M80 in a 55 gal drum there is less pressure than if you throw it in a gallon paint can.

And please don't respond with a formula that says it depends on the brand of paint.

Good but show work.

[njl]

Good lord!! I didn't take any of those courses but I know if you throw an M80 in a 55 gal drum there is less pressure than if you throw it in a gallon paint can.

And please don't respond with a formula that says it depends on the brand of paint.

Maybe not so much the brand of paint, but what else is in the drum or paint can could make a huge difference.

[Sarge]

Good lord!! I didn't take any of those courses but I know if you throw an M80 in a 55 gal drum there is less pressure than if you throw it in a gallon paint can.

And please don't respond with a formula that says it depends on the brand of paint.

Good but show work.