Powder Burn and Pressure Relationship

[Doc Highwall]

popper, the trace I posted above shows where the bullets left the barrel where the + is on trace at about 1.3 milliseconds. There is also a lot more data given that I did not post like barrel temperature, barometric pressure, area under the curve.

[303Guy]

On those five traces, note the difference in time until muzzle exit. There's no way of calculating velocity from them, the barrel times are essentially the same with exception of the first and lowest pressure trace. What's interest to me is the shape of each trace. Those show the relative rate of rise of pressure and relative muzzle pressure.



Now this is where it gets suspicious. The secondary spike is occurring after the bullet has left the muzzle!

Now here's a little bit of information to think on - for that high pressure spike to register on an externally mounted strain gauge means the chamber over which it is mounted is expanding very rapidly indeed. And it doesn't crack the epoxy gluing it down? I coated a barrel with polyester resin - the type used in fibre glass reinforced resin - blew off with the first low pressure shot! Besides, a strain gauge is an electrical device which means it will respond to an electrical pulse. I have a calculator mounted on the loading bench which makes beeping sounds in response to touching a boolit against a transformer terminal even when that calculator is switched off. The strain gauge which is made of the same stuff as the internals of that calculator with silicon chips and connecting wires. A bullet being driven down the bore must surely develop a significant electrical charge. That's only one possibility, as unlikely an explanation as it may be. Just think, an extreme pressure excursion occurring after the bullet exits the barrel in one case and before exit in another yet no primer rupturing or excessive flattening, cratering or whatever is reported in either case (six cases actually). Then again I've mentioned before about a case getting stuck by the neck expanding into rust pits in the chamber while the primer shows nothing at all. There was probably a localized pressure spike at the boolit base. None of those boolits were recovered for base examination but they did shoot accurately.

[waksupi]

Would the sudden pressure back into the barrel cause the second spike? A bullet exiting the tube must create a hell of a vacuum for a millisec. I would think it would be quite abrupt, and cause a secondary discharge of thrust.
I haven't had time to keep up on this interesting thread, just tossing out ideas.

[Doc Highwall]

In post #102 by 303guy, this is what was said in regard to the pressure trace for Nosler 7-STW Factory ammo.

This interesting pressure trace was taken while shooting factory ammo. It was surely tested using standard SAMI protocols but if the lab used a peak capture meter, not an oscilloscope, they would not have seen the severe secondary pressure "if" it occurred in their test barrel.

The exit time markers are where they should be for a standard barrel. Secondary pressure events are typically less severe and INSIDE the end of a long barrel. (See tech article.)

We believe this to be a gross example of "deflagration" or unburned powder burning AFTER the bullet exits. The scary bit is more than 81,000 PSI is read by the strain gage located over the chamber! It is impossible to know if the entire length of the barrel was exposed to this pressure. But if it was, then the load is approaching the yield strength of thinner parts of the barrel and "could" expand the bore.

Ironically the exit timing, velocity S.D. and accuracy was great for factory ammo... but it does light up half the county when fired at night.

[Larry Gibson]

Larry, the strain gages that come with the RSI unit have a gage calibration factor to input into the program.

Could be the same gauges used for the Oehler?

Larry Gibson

[Larry Gibson]

Doc

I don't think those are really "secondary pressure spikes".. I had similar ones once and called Dr. Oehler. He asked several questions regarding the physical set up of the equipment and the bench used. When I moved the equipment to a more prtected psition from the muzzle blast and the shock vibrations created by the bench to the equipment the spikes, similar to yours, went away. Only happened with some guns, cartridges and loads.

The secondary pressure spikes I refer to occur before muzzle exit. Equipment set up has no effect on those.

Regarding psi along the barrel; You can graph the time/pressure curve (trace) and get a real close estimate of the psi at any point in the barrel.

Also note; peizo/transducers also do not measure the psi "directly" as the cartridge case is expanded by the psi to push against the transducer which converts the pressure of the push to electrical impulse which is then "translated/converted to psi by the computer program. Pretty much the same as the strain gauge. Dr Oehler conducted a test where he had a C.U.P. device, 3 different transducers and 3 different strain gauges (hooked to a M43 and M83s). He could then compare all those systems for each test shot fired as they all measured the same psi/trace for the same test shot. The results show the accuracy of the strain gauges compared to the C.U.P. and transducer methods.

Larry Gibson

[Doc Highwall]

Larry, go to this page and you will see the secondary pressure spike before bullet exit.

http://shootingsoftware.com/

[popper]

Attachment 67474
Interesting dip in pressure on this one, not sure what causes it. Ideas? Most of my traces have a pressure divit toward the muzzle. I won't go into reflected wave theory. Most of my guns have a tight bore at the muzzle. Load cell is usually a piezo between the test device and a reaction block (relatively large mass) to directly measure pressure.

[Larry Gibson]

Larry, go to this page and you will see the secondary pressure spike before bullet exit.

http://shootingsoftware.com/

Interesting! A test I ran of Winchester white box M193 gave 65K+ psi with a correspondingly high velocity. I gave up after 5 rounds. It also gave indications of a secondary spike but the test barrel was 21" so perhaps that negated it(?).

Larry Gibson

[Larry Gibson]

popper

I see lots of traces like that; usually associated with medium or slow burning powders on the low end of consistent ignition and burn.

Larry Gibson

[DrCaveman]

I know this is a little off topic of where this thread has gone, but i think it might be a useful aspect of the ultimate conclusion.

Im reading Sixguns for the first time, and in the chapter titled 'cartridge reloading' Keith states a balance point for several powders. bullseye and unique are the ones i use, he also mentions dupont #6 and #80. Not sure what today's equivalents are for those two, but someone here surely knows.

He states that bullseye has a balance point of 9000 psi, extending 4-5000 pounds below and 3000 pounds above. Unique has balance at 13000 psi, 4000 pounds above and 'very little below'.

Taking for granted that he is right (and my respect increases with the more of the book i read).....

Interesting that two popular powders, used in a lot of the same cartidges, would offer this disparity in performance. Simply put, and common knowledge to most: unique doesnt shoot as well when downloaded, bullseye does well from max loads down to target loads. He is just putting numbers to this phenomenon.

What i anxiously awaited in the book was a balance point stated for 2400. I never saw one. Why not? There must be an ideal pressure, then a low and high extreme for acceptable performance. This number should be related to powder charge, cartridge, and seating depth, of course. But also probably will be influenced by barrel length and even maybe boolit fit.

Seems that he was talking about complete ignition of the powder, 100% combustion in a perfect scenario which will never actually happen but you can get close. A 'balance point' and edges of the curve must exist for all these powders, right?

No unburned powder means no secondary spikes?

Maybe not so simple because various barrels will be more accurate at certain velocities, regardless of perfection of powder burn.

Maybe this is not helpful at all. But i am curious what the 'balance points' are for all these powders out there. Loading manuals should have that list

[Doc Highwall]

You can have a secondary pressure spike and still not leave any un-burnt powder in the barrel. With short barrels I would doubt you could see a secondary pressure spike as the un-burnt powder would be blown out the barrel with the bullet and is probably what makes for a large muzzle flash.

[303Guy]

That's an interesting aspect, DrCaveman. Thanks for introducing it. I've been thinking some vague thoughts along those lines and now you've clarified them. And I agree, the loading manuals should have a list of 'balance points' for the powders we use - make that all powders.

An observation I've made is that a powder won't burn well at low loadings which equates to low pressure then burns well as the loading goes up then reaches a zone where pressure begins to go north fast. Thinking of case capacity and powder charge, I keep hearing about the 30-06 doing well with cast using Unique and I'm thinking of a balance between powder charge and case capacity and given the relatively low expansion ratio of the 30-06, the generated pressure will be maintained for a fair distance down the bore producing reasonable velocity with a low peak pressure. I'm referring to initial volume over final volume. A higher expansion ratio rifle would not be able to achieve those velocities with such low pressures and perhaps not allow the time for consistent burn.

I have a heap of Bullseye equivalent and find it too fast for the Brit. Likewise I find Trail Boss too fast but at lower loadings things seem to improve. My suspicion is that all the powder burns before the boolit moves vary far so case capacity is all there is to absorb the hot gasses formed. This is where my fuzzy idea came in - the case needs to be able to contain the gasses formed by combustion before expansion effectively begins. Too little powder produces too low pressure and too much produces a pressure spike. The balance. Part of that balance might include the boolit mass or weight. A lighter boolit seems to be better for a smaller case with faster powder.

If I had a pressure trace machine I could have a lot of fun finding the balance for different powders and boolit weights!

[Doc Highwall]

303Guy, look at my post page 3, post #44 showing the bore volumes for equal length barrels and how it relates to pressure drop off.

[popper]

There's no way of calculating velocity from them

Yes, there is. Area under the curve is energy. The velocity can be calculated from the mass and energy. I read somewhere that this energy is divided into propelling the boolit, heating the barrel, frictional losses and expanding the barrel. The source indicated it was somewhat caliber and powder independent. The powder burn rate determined the chemical efficiency of creating the pressure.

[felix]

You read that here, Popper! ... felix

[popper]

Felix - World's Great Rifles, by Roger Ford (1998, Brown Packaging Books, Ltd., London). 20-30% goes to fps, 40% muzzle blast, 3% friction, 30% heating(and expanding the bbl).
Don't know his source.

[felix]

Yeah, that would be a good (and expected) average to quote using normal components (including gun). The statistics would change somewhat if StarWar plastics were involved giving more energy directly to the projectile. ... felix

Information probably came from such studies via JohnsHopkins performed for the Navy between the big wars. ... felix

[popper]

I was amazed at the 70% being 'wasted' as we would consider. Thinking about it makes sense though, Muzzle blast is the hot gasses that don't do any work on the CB ( but do on the air outside). Another comment from that source was that powder gives ~ 180 ft-# per grain, properly burned. Also, twist uses 0.2%, but I assume this is a slow twist value. I've read about the amount needed to engrave but forgot that value. It wasn't too high, even for jacketed. I've over estimated friction, under estimated bbl energy. Anybody have the Powley equations?

[303Guy]

I'm surprised at the value given for heating the barrel (and expanding). The barrel doesn't get that hot from each shot. Barrel expansion is elastic so does not retain much energy but I suppose the energy it does absorb is only released after muzzle exit. Nothing said on recoil energy?

I've had bullets stop in the bore only to almost drop out once cooled - the jackets showed heat scorching, indicating lack of thermal contact with the lead core (those bullets never shot accurately with full power loads).

I've had a hornet load that heated the barrel as much as a 303 does. Velocity was pretty high. I increased the powder charge a small amount and that heating went away. Velocity went up but I don't know by how much - noticeable on the range in that it was slightly flatter shooting. That was using Lil'Gun.

Oh, how does one differentiate frictional energy from barrel heating energy? Frictional energy is heating energy!