question on internal ballistics

[Black Jaque Janaviac]

I'm wondering if there are any sources of information on internal ballistics that might give me some idea of when various powders reach peak pressures vs. how far along in the throat and/or bore the bullet is at that point.

Can anyone point me to some resources?

Or perhaps some people here know about these things.

Does peak pressure occur while the base of the bullet is still in the case? Still in the throat? Or somewhere down the bore?

Can this depend greatly on the burn rate of the powder?

I'm thinking this could have an impact on what powders will perform well with cast bullets. Is it possible that leading can be prevented not only by playing with alloy hardness & lubes, but also by using different powders/burn rates? I'm thinking if a slower powder doesn't generate the pressures needed to cause blowby & leading until after the bullet has effectively sealed the bore, one might be able to shoot faster with softer alloys.

[AZ-Stew]

Your logic is correct, but there is no way to predict exactly what the peak pressure will be or how far down the barrel the boolit will be in any particular firearm. Too many variables.

Boolit weight
Boolit bearing surface
Boolit hardness
Powder type and burn rate in the cartridge in which it's being used
Primer brissance and power
Variations in tolerances in individual firearms
Ambient temperature of all components and firearm

And probably a dozen other things I haven't listed.

We've discovered by experimentation that some powders work better than others with cast. This depends on the type of firearm (rifle, handgun) and the desired velocity. The load tables in various cast bullet handbooks will give you some clues.

Regards,

Stew

[Black Jaque Janaviac]

So, the descrepancy between Bullseye and H110 is so narrow that there's no way to tell?

In one gun a Bullseye load might peak when the bullet is well into the bore, whereas in a different gun the H110 might peak before the bullet base has even left the case mouth?

I didn't think it would be that bad. I thought there would be some overlapp but that perhaps the various burn rates would have a "window". For example; that maybe Bullseye would peak somewhere between when the bullet base is still in the case but generally before the bullet base leaves the forcing cone. Something like that.

[AZ-Stew]

I think your second paragraph has the powders reversed, but generally speaking, you're right... with one caviat. The condition you're describing might exist in one particular cartridge (.357 Mag, .45 Colt, etc.) and in one particular firearm. But don't expect these condition to carry over into another firearm.

For instance, various revolvers, even within the came caliber, have different cylinder lengths. In a revolver with a relatively short cylinder, a slow powder may not completely burn until the bullet has passed into the barrel. On the other hand, in a gun with a long cylinder may burn it all within the cylinder. I'm thinking, for instance, .45 Colt fired from a Ruger Blackhawk using 2400 powder, versus the same load being fired from a Taurus Judge., which will fire a .410 shotshell (long cylinder).

What's the practical result? I don't think it can be accurately predicted. The results can, however, be determined and adjusted to the shooter's needs through experimentation, which is why we're here and why we love this pursuit.

Regards,

Stew

[Black Jaque Janaviac]

That's kind of what I meant by the second paragraph. I thought that what you were telling me is that there is so much variation with other variables that conditions can be entirely reversed simply by changing boolit weight, length, hardness, gun, ambient temp etc.

I would guess that the points at which peak pressure occurs would overlap based on those variables when comparing powders of close burn rates such as Red Dot vs. Green Dot. But I thought maybe pressure peaks would vary enough that the outside variables would matter less when comparing Bullseye to H110.

I guess I'm not looking for a way of predicting what each and every powder will do but I think it would be useful information to help in trouble shooting.

For example, if you're developing a load for a specific purpose but you're having problems. It might be helpful to be able to determine if the cause of the problem is because the bullet is obturating in the case mouth, chamber throat, or bore. Or whether gas was blowing by at one of these points.

[Willbird]

One thing I read a few years back lead me to believe that 38 special reaches peak pressure before the bullet ever leaves the cylinder. This was when S&W showed off how strong their TI snubby was by threading in a barrel stub with no hole it it, yup just bar stock, and firing 5 rounds with no damage to the gun.

I have also read some tests a guy with with a 1911 where he had a fixture that HELD the bullet in the case of a 1911, no bullet movement was allowed, he fired a shot, the case did not blow down the magwell and in fact showed no visible signs of excess pressure.

These things lead me to believe that many revolver and semi auto cartridges experience peak pressure before the bullet even fully leaves the cartridge case. very unlike high powered rifles where peak pressure is down the barrel a few inches.

Bill

[Black Jaque Janaviac]

very unlike high powered rifles where peak pressure is down the barrel a few inches

That's good info! Is this true? How many inches?

Hmm. I can't quite figure out how the pistol examples you cited prove that pressures peak before bullets leave the throat/cylinder. In both examples you mention that the bullets weren't even allowed to move beyond that point. Well then, of course the pressure is going to peak before the bullet leaves.

[AZ-Stew]

Willbird's comments are valid, but again, it depends on the powder and cartridge. I've had my hand peppered with unburned kernels of 2400 escaping from the barrel/cylinder gap of a revolver. Was combustion still in progress and these just got blown out before they could ignite, or were these powder kernels never going to burn? I don't know any way to tell.

If you have a means of measuring pressure (the discontinued Oehler Personal Ballistis Lab would work), you can estimate the position of the bullet at peak pressure in a rifle, but I don't think you can determine it exactly.

Regards,

Stew

[Willbird]

That's good info! Is this true? How many inches?

Hmm. I can't quite figure out how the pistol examples you cited prove that pressures peak before bullets leave the throat/cylinder. In both examples you mention that the bullets weren't even allowed to move beyond that point. Well then, of course the pressure is going to peak before the bullet leaves.


OK to qualify my statement, the pressure created by blocking the bullets passage is not higher by a magnitude great enough to blow the gun up. On the rifle deal it is strictly from reading over the years that I have that impression. Many guns will blow unburned powder out the muzzle, shooting a target at 4-5 feet range with 00 WW 12 gauge buckshot will pepper drywall with holes from unburned or still burning powder. Also in actual tets I have read about over the years FAST powders did not show a higher velocity in shorter barreled pistols than SLOW powders do, in fact generally the powder that gives the highest velocity in an 8-3/8 will also show the highest velocity in a 1-7/8" barrel of all powders tested.

Bill

[John Boy]

BJJ - read this ... http://www.shootingsoftware.com/pressure.htm
The strain guage and accessories is expensive though
The Entire RSI Shooting Lab With Full Instrumentation For Windows $757.95

[Willbird]

BJJ - read this ... http://www.shootingsoftware.com/pressure.htm
The strain guage and accessories is expensive though
The Entire RSI Shooting Lab With Full Instrumentation For Windows $757.95

There is a book called "Rifle Accuracy Facts" by Dr. Harold Vaughn. It is avail from amazon and Sinclairs used to have it to.

He does a lot of interesting things in the book, including showing how to use a strain gauge with an oscilloscope (how to calibrate it too),

how he used accelerometers mounted on the rifle barrel to measure the barrel whip while the bullet went from breech to muzzle....and a zillion other things related to how guns work.

It is an interesting read.............if you got a scope that would handle more than one input you could mount several strain gauges on a barrel and measure the pressure at that point.

Bill

[Black Jaque Janaviac]

Forgive my weakmindedness, but I can't understand how unspent powder serves as much evidence.

Here's the way I understood the behavior of a progressive powder:

powder ignites, pressure builds, powder burns faster, bullet moves, pressure peaks, bullet continues to move, volume of space behind bullet increases, pressure drops. Now depending on the burn rate of the powder it may either be entirely consumed before the pressure drops. However if it is not entirely consumed before the pressure drops it might not ever get burnt.

Burnt or not burnt the pressure still peaked somewhere before the boolit left the muzzle.

Actually, come to think of it, I do have a recovered paper patch boolit that actually has a flared base on it! It was fired from a 6.5" Ruger Blackhawk using Lil' Gun. Now I suppose this doesn't necessarily tell me the point of peak pressure, but it does tell me that the pressure at exit was still well beyond that needed to obturate a soft lead slug. Or am I off base?

[Larry Gibson]

Black Jaque Janaviac

My pressure tests using a M43 Oehler hooked to a 8.4" Contender .44 magnum barrel show that the pressure peaks at about 2.75" down the barrel with 2 loads; the 240 gr XTP over 24.5 gr H110 and the RCBS .44-250-K over 22 gr Alliant 2400. How that would correlate to a revolver I do not know.

I have not pressure tested any loads with Bullseye but could in the not too distant future.

Larry Gibson

[Willbird]

Larry,

How do you determine with the Oehler where the bullet is in the barrel at any time on the pressure curve ??

Bill

[Larry Gibson]

Larry,

How do you determine with the Oehler where the bullet is in the barrel at any time on the pressure curve ??

Bill

Part of the data printout for a test string is a time pressure curve (like on oscilliscope screen) for each shot fired. The line starts at ignition and measures the time under the curve. The M43 places a mark on the graph line where the bullet exited the barrel. You simply measure the total time under the pressure curve. Measure the total time to peak preasure and figure what percentage that is of the total time under the curve. Then take that percentage of the barrel length and it tells approcimately where in the barrel the peak pressure occured. Harder to explain that to do.

Larry Gibson

[Boerrancher]

Part of the data printout for a test string is a time pressure curve (like on oscilliscope screen) for each shot fired. The line starts at ignition and measures the time under the curve. The M43 places a mark on the graph line where the bullet exited the barrel. You simply measure the total time under the pressure curve. Measure the total time to peak preasure and figure what percentage that is of the total time under the curve. Then take that percentage of the barrel length and it tells approcimately where in the barrel the peak pressure occured. Harder to explain than to do.

Larry Gibson


I got it Larry but I think it would have been easier to get and easier to explain had you posted the actual mathematical formula, and your are correct it is harder to explain than to do.

Best wishes from the Boer Ranch,

Joe

[Black Jaque Janaviac]

Part of the data printout for a test string is a time pressure curve (like on oscilliscope screen) for each shot fired. The line starts at ignition and measures the time under the curve. The M43 places a mark on the graph line where the bullet exited the barrel. You simply measure the total time under the pressure curve. Measure the total time to peak preasure and figure what percentage that is of the total time under the curve. Then take that percentage of the barrel length and it tells approcimately where in the barrel the peak pressure occured. Harder to explain that to do.

OK. So if you shot a 20-inch bbl and the peak occured at 50% of the duration of the pressure curve, then 50% of the 20-inch bbl would be 10 inches down the bore. Correct?

The fact that the bullet is going faster by the muzzle compared to the breach doesn't have an affect on this formula? In otherwords it should take less time for the bullet to cover the last ten inches of barrel than it does the first ten.

[Willbird]

OK. So if you shot a 20-inch bbl and the peak occured at 50% of the duration of the pressure curve, then 50% of the 20-inch bbl would be 10 inches down the bore. Correct?

The fact that the bullet is going faster by the muzzle compared to the breach doesn't have an affect on this formula? In otherwords it should take less time for the bullet to cover the last ten inches of barrel than it does the first ten.

I agree, any guess on bullet position would be just that, a guess :-) ??. An Educated one however :-)

unless you used a series of strain gauges every 2" apart

[Larry Gibson]

Black Jaque Janaviac

"So if you shot a 20-inch bbl and the peak occured at 50% of the duration of the pressure curve, then 50% of the 20-inch bbl would be 10 inches down the bore. Correct?"

Basically correct.

"The fact that the bullet is going faster by the muzzle compared to the breach doesn't have an affect on this formula?"

No it doesn't. You asked where the bullet was in the barrel when peak pressure was reached. The additional velocity gain has nothing to do with that loacation of the bullet.

"In otherwords it should take less time for the bullet to cover the last ten inches of barrel than it does the first ten."

That is correct; after peak pressure is reached the bullet is continuing to accellerate even though pressure is decreasing, thus the last half of barrel travel is faster than the first half.


Larry Gibson

[Larry Gibson]

I got it Larry but I think it would have been easier to get and easier to explain had you posted the actual mathematical formula, and your are correct it is harder to explain than to do.

Best wishes from the Boer Ranch,

Joe


Half of it is taking measurements off the graph which is printed to scale.

Larry Gibson

[mike in co]

quick load does provide time and pressure graphs.
it would not be specific data for your gun, but it would be data that the developer generated from his testing and then applied to the conditions you enter.
i would expected it to be close.
mike in co

[Cap'n Morgan]

"So if you shot a 20-inch bbl and the peak occured at 50% of the duration of the pressure curve, then 50% of the 20-inch bbl would be 10 inches down the bore. Correct?"

Basically correct.

When I run a simulation in Quickload, a 240 grain bullet in a 44mag, in front of 20 grain 2400, will only travel about .5" before reaching peak pressure. This will happen at about 0.2 ms. About 0.5 ms later the bullet will leave the muzzle (9" barrel) At half barrel time (0.35 ms) the bullet will have moved about 2".
This means that the bullet covers the last six inches in the same time as it did the first two.

[Larry Gibson]

When I run a simulation in Quickload, a 240 grain bullet in a 44mag, in front of 20 grain 2400, will only travel about .5" before reaching peak pressure. This will happen at about 0.2 ms. About 0.5 ms later the bullet will leave the muzzle (9" barrel) At half barrel time (0.35 ms) the bullet will have moved about 2".
This means that the bullet covers the last six inches in the same time as it did the first two.

Perhaps that's the difference between 'simulation" and actual measurement. Most any actual measurement will show the peak pressure to be farther down the barrel than that.

Larry Gibson

[leftiye]

IIRC correctly, some of the computer internal ballistics calculators (quickload?) give position of pressure peak also on estimated loads. In Larry's case the defining piece of information is that the pressure curve printed out is to scale (just measure it). Otherwise, the issue of the boolit accelerating would make it necessary to use calculus to get more exact if only time values were available and velocity at exit.

Way back when I started reloading, there were some tests of this nature done, and there were a lot of the medium pistol powders which never peaked in the barrel (say 5" barrel), a longer barrel would have to have been used to find the peak position. IIRC this was with semi auto (probly 9mm/ 38 caliber range) pistols.

I had a .38 super that I worked up loads with and went waaaay over maximums. The pressure signs peaked and then backed off as I increased the charges! My guess is that as the velocity went higher and the pressure peak moved out of the barrel, so that the internal pressure didn't increase.

[Cap'n Morgan]

I think we're talking apples and oranges here.

What Larry's readouts show is the pressure/time curve - and the pressure will roughly peak about 1/3 along that curve, but this doesn't indicate the bullet has traveled 1/3 of the barrel length. Time is a constant, but velocity is not - when pressure peaks at 0.3 ms the velocity is only about 500 fps and the bullet has moved less than half an inch.

[Black Jaque Janaviac]

From what I understand of an accelerating object, Capt. Morgan is making the most sense. How close the numbers in his example are to the truth I have no idea.

If it takes longer for the bullet to cover the first half of the bore than it does to cover the second half, then that can only mean that it more than 50% of the time is spent covering the first half of the barrel. Thus taking the percent of time-to-peak and equating that to bullet position does not make sense.

[Willbird]

You could get some data to help estimate by firing a revolver without a barrel, that would give you SOME idea what velocity the bullet has when it leaves the cylinder. Also if a strain gauge is placed on a revolver barrel, it misses part of the pressure curve before the bullet gets to the strain gauge.

Bill

[Larry Gibson]

The time pressure graph of the Oehler M43 shows the time of ignition, when pressure overcomes the strength of the case to press against the barrel. As previousl stated it is to scale. The other thing to remeber is I used a Contender barrel; no cylinder throats and no barrel/cylinder gap. The first of those means the peak pressure will rise quicker be cause of the resistance to the rifling vs the "smooth bore" of cylinder throats. The second means pressure is bled off by venting out the barrel/cylinder gap. What the total effect of both is I do not know. Additionally the 2.75" "down the barrel" of a Contender (measured from the breach face) is, of course, a different "down the barrel" in a revolver (revolver barrels are measured from the face of the cylinder to the front end of the barrel). That is why I stated I did not know how that data would correlate to a revolver. It is obvious some here are misconstruing those facts.

However, for sake of discussion, if all else was equal with that test in my Ruger BlackHawk then the peak preassure would occur with the bullet about .9" into the barrel of the revolver. That would leave 5.6" of barrel (the Ruger has a 6.5" barrel) for the velocity to continue to increase as the pressure falls off from the peak pressure. This is all hypothetical based on a simple comparisong of the Contender barrels actual measured time/pressure curve and the peak pressure vs that we "assume" it will be fairly the same in the revolver. At any case; this indicates to me that with slow powders in magnum revolvers the peak pressure will occur very shortly after the bullet has entered the barrel. I would imagine that when loaded with fast burning powders that peak pressure may occur before the bullet enters the barrel of the revolver. However, testing will have take place before I state that as fact.

Larry Gibson