Awesome shock wave pics: .44 and .30-06

[atblis]

the velocity of the gasses can't be higher than the bullet at exit...a 6 grain powder charge at 1,000 fps would generate a recoil impetus roughly equal to a CB cap.

Notice that the molecular speed is a distribution. There isn't a hard limit, just probabilities.
http://hyperphysics.phy-astr.gsu.edu/Hbase/kinetic/kintem.html#c4

 

[1911Tuner]

Notice that the molecular speed is a distribution.

Okay...I'll concede to that. The ejectiva can move faster for a brief instant around the bullet base at exit.

So...6 grains at 2,000 fps would provide..what...about half the impetus of a standard velocity .22 Short?

 

[atblis]

Yeah

I would also guess not much.

However

So...6 grains at 2,000 fps would provide..what...about half the impetus of a standard velocity .22 Short?

I am guessing 2000 fps is a bit low. We're quickly headed into something similar rocket science here.

Rough values thrown into that calculator above yield 15,000 fps for the mean velocity.

 

[1911Tuner]

Rough values thrown into that calculator above yield 15,000 fps for the mean velocity.

Nah. The pressure and volume isn't there in a straight-walled pistol case. Magnum revolver calibers or a bottleneck rifle case in the .308 or .223 class...probably that high.

 

[brickeyee]

Mass requires force to accelerate it to a given velocity. The velocity of the gas can't be higher than that of the bullet while the bullet is blocking its path of escape.

Once it's unconfined, the mass of the gas can't accelerate to a higher velocity unless a new force is introduced any more than a bullet can accelerate to a higher velocity after leaving the barrel simply because it's free from the barrel's frictional resistance. No?

The driving gas behind the bullet is still at very high pressure compared to the atmosphere.
Once the base of the bullet clears the barrel that high pressure is released and the gas accelerates into the atmosphere.
Since it has a low density and is expanding in every direction it slows very quickly.
Why does the gas come out of a steel cylinder when we open the valve?

The ejecta weigh exactly what the powder did before ignition.
Atmospheric oxygen does not play a part in internal ballistics.
Once the hot gases enter the atmosphere they will react with the oxygen and nitrogen to finally complete the combustion process to stable ambient pressure and temperature compounds.

If you run through all the chemical reactions going on it would be better if the powder simply decomposed to hot mono atomic gases.
Cooling and further reactions actually decrease the energy available from the powder gases to do useful work on the bullet.

 

[atblis]

What volume and pressure would be agreeable to you? Let's assume the bullet is at the end of the barrel but not out?

Atomic mass is another issue. Anybody have any insight into what sorts of compounds would be present?

 

[brickeyee]

What volume and pressure would be agreeable to you? Let's assume the bullet is at the end of the barrel but not out?

It is still thousands of PSI, and even higher for rifles.

I just ran QuickLOAD with 8.0 gr of AA#5 and a 230 gr FMJ.
With a 5 inch barrel length the pressure is over 3000 PSI when the bullet leaves the muzzle.
A .22-250 AI with 41 gr of IMR4895 and a 40 gr bullet shows over 14,000 PSI at the end of a 26 inch barrel (and right around 4,000 ft/s).

Once you ‘uncork’ the pressure vessel (bullet leaves barrel) that gas expands into the ambient air.
It was pushing the bullet before, now it has essentially no obstruction to its free expansion.

It is a few hours of work to go through all the analysis with the Explosive Engineering Handbook (Paul Cooper) and even attempt to compute the products of decomposition for nitrocellulose and then estimate the final state of the reaction products.

The reactions do NOT typically go to completion with simple balanced chemical equations. Welcome to the nasty world of organic chemistry.
This is not combining H2 with O2 to create water (and a bunch of heat).

Cellulose nitrate is C6H7(NO2)3O5 and it is then esterified to produce a stable material.

Everything from mono atomic nitrogen, hydrogen, water, ammonia, and various nitric oxides (among others) will be produced.
Add to the straight nitrocellulose products the deterrent coatings, left over solvents, graphite (carbon), primer compounds, etc., and temperatures well into the 4000F and up range and pressures from 20,000 PSI to over 60,000 PSI and the gases out the muzzle are a real witches brew.
These hot gases will further react with the oxygen and nitrogen in the atmosphere (producing more muzzle flash) until they reach a final state and cool off.

 

[1911Tuner]

It is still thousands of PSI, and even higher for rifles.

I'll have to disagree...at least in part.

Pressure peaks early...even in rifles.
As the bullet moves through the barrel, the increasing volume of the cylinder behind the bullet causes pressure to drop rapidly....again, depending on the burn rate of the powder and the area under the curve.
The slower the powder and the fatter the curve, the higher the eixt pressure will naturally be.

A quick pistol powder wouldn't likely have more than 2 or 3 thousand psi remaining at the point of bullet exit.
It's even been shown that too-quick powders in long rifle barrels produce lower velocities than the same powders with less barrel length. The pressure is too low to keep accelerating the bullet...and barrel friction starts to slow it down.

Let's assume the bullet is at the end of the barrel but not out?

The gas plug behind the still captive bullet would have a hard time outrunning the bullet...especially seeing as how the pressure is falling with every millimeter that the bullet travels. Once the bullet is just past the point of exit...say a couple thousandths of an inch...I agree that the "nozzle" effect would tend to let the remaining pressure accelerate a small volume of gas for a brief instant.

Even if we accept that the gasses are traveling twice as fast as the bullet before it exits...it requires a helluva lot of force to accelerate even 6 grains of a mass from 2,000 fps to 15,000 fps during the brief time frame provided by the "nozzle" of the bullet base and the muzzle.

I'm sorry, but I'd have to see the gas velocity actually measured before I could believe that it could be 15,000 fps in a pistol that started with 5 or 6 grains of powder. I can possibly accept it with a rifle caliber...but not a pistol.

I believe the expansion rate of C-4 is somewhere around 20,000 fps...and it's a lot quicker than any smokeless propellant.

 

[brickeyee]

You need to play with a program like QuickLOAD.
It plots pressure and velocity as a function of either time or bullet travel.
While the volume is expanding as the bullet moves, Charles law is at least roughly applicable.
You have to double the volume to cut the pressure in half.
A 20,000 PSI peak that occurs just before the bullet clears the case will drop to 10,000 after about an inch (remember you have to include case volume as the 'starting' volume and we are neglecting continued decomposition of the powder).
Now the bullet has to move twice as far to drop the pressure to 5,000 PSI.
And twice as far again to drop it to 2,500 PSI, but you just ran out of barrel in a 1911.
By adjusting the powder burn factors I can make QuickLOAD exactly match a load of powder and my guns.
It becomes painfully accurate for muzzle velocity at this point.

 

[1911Tuner]

Fellers...I ain't doubtin' that mollecular transfer thing...and I might be wrong about it...but when the free expansion rate of C-4 is 25-26,000 fps...(I looked it up)...which means that nothin' is in its way except air......and I'm supposed to accept that 5 or 6 grains of pistol powder is gonna hit 15,000 while it pushes a bullet through a barrel...and in a centerfire rifle, it can equal or exceed that rate while it pushes a bullet...we can probably argue this one 'til Christmas without gettin' any further than we are now.

I just don't think that dog'll hunt until I see it measured on an instrument that's designed to measure gas velocities under pressure.

 

[chris in va]

We've all seen this, but you can see the gasses coming out before the bullet.

http://youtube.com/watch?v=BW93WQ98s-I

 

[brickeyee]

C-4 has a detonation velocity of ~24,000 ft/s.
That is how fast the explosive front moves through the material.

The gases behind a bullet are simply expanding until the bullet leaves the barrel.
Once the bullet exits the pressure falls to essentially zero and all the remaining energy is converted into speed.
There is a reason the gas comes out of a cylinder at very high speed if you just crack the valve open.

 

[benEzra]

Atomic mass is another issue. Anybody have any insight into what sorts of compounds would be present?

I believe molecular hydrogen, carbon monoxide, and molecular nitrogen are among the combustion products until they hit atmospheric oxygen after exiting the barrel.