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Although not specifically mentioned, the charts and graphs illustrated seem to relate to a certain case capacity, bullet weight and diameter and barrel length.

Said another way, a powder type balanced with the cartridge and caliber and bullet weight.

My question is, if we take it to the extremes and use a much lighter bullet, say 100 grains in a .30-06 and use the largest amount of the slowest burning powder that is safe to use, would all the powder burn before the bullet exits the muzzle?

Would it all burn within the first 5 or 6 inches of bullet travel.

It seems to me, to get the most velocity from a given cartridge, you would have to balance the powder burn rate with the bullet weight and case capacity.

The part about the pressure dropping after peak pressure. That is because the bore capacity is increasing as the bullet moves forward.

There is a name for this, but I can't think of it. It goes something like this--you have a sealed container with X amount of gas pressure inside it. You double the size of the container, and the gas pressure drops to 1/2X. If you half the size of the container, the gas pressure increases to 2X. This pressure rise and/or fall is also progressive and at a constant ratio.

That is the reason the gas pressure drops after it peaks in a barrel and the powder is no longer burning and producing gas.

But, as long as the pressure inside the barrel is greater than the pressure outside the barrel, the bullet will continue to accelerate.

That can also be what causes a gun to blow up. If the bullet is not moving forward fast enough to create more bore volume to keep the gas pressure in check, the gas pressure will soon overcome the available volume, pressures will rise, and if they rise high enough, something will let go.


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2525,

It is a common misconception that the mere increase in bore volume versus the amount of gas is the total cause the curve drop off.

But why does it drop off so quickly? If mere bore volume was the answer the curve would drop more gradually, instead of at a definite peak. The peak (or just past it) is where powder has ceased to burn to any significant extent.

Last edited by Mule Deer; 09/25/09.

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And to add to JB's words, the "tailoff" as I (and rocket engineers) call it is just what 2525 said: the last bits of propellant kernels burning out. There are always some that are slower than the others. A small percent of the powder can ignite late. The higher the loading density, the tighter packed the charge and the greater the distance from the primer, the larger that percentage can be. And some powder, as observed above, never lights at all.

But I think that the real experts in the field are almost unanimous that effective burning is signaled by the pressure peak. And I probably could have left out that "almost."


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I just wanted to comment on the "charts and graphs" that somebody mentioned being shown here. There has only been one, showing a .30-06 with a 150-grain bullet and Varget. It is not anywhere near a maxiumum load, in fact the peak of any of the curves is around 44,000 psi, which is far below the peak (and most efficient) pressure for Varget, or many other modern rifle powders.

The pressure curve at 55,000-60,000 psi with most modern rifle powders shows a much steeper climb to the peak, and a steeper drop-off. I have looked at a bunch of Pressure Trace curves and at 55,000 to 60,000 psi the curve has a much more obvious peak.

So the "charts and graphs" are not plural, and the single one shown does not represent a typical load for modern bolt-action rifles.


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"It is a common misconception..." JB, I must disagree that near peak pressure the rate of change of volume is insignificant. QL is predicting that at peak pressure, the bullet is already up to 1000 fps. That is 12 inches per msec which works out to an increase in volume of about triple the case capacity of the .30-06 every msec. The pressures in the RSI trace are above 90% of the peak for the .25 msec or so centered about the peak, so the volume increase is about 3/4 of a case during that time. This rate of volume increase coupled with decreased grain size remaining seems perfectly consistent with QL's figuring that the burn simply can't keep up with the bullet.

You ask why does the curve drop off so quickly, yet I ask why doesn't it drop more quickly. My claim is that if the powder were fully burned by peak, then the curve should fall off more sharply. Instead, we see a big dome that's roughly centered around the pressure peak.

As for a 60 ksi trace, had I one at hand, I'd be happy to look it over. That one on their web site was the first I came across. I'll see if I can't find one on their site tomorrow. I'm heading out for the evening.


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Stop you makin me dizzy!! LOL


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I have seen a bunch of 60,000 psi Pressure Trace graphs, created when I was using one. The peak goes up much more rapidly and drops of more quickly as well.

I am out of this discussion. This point has been proven by professional ballisticians over and over, especially Homer Powley at Picatinny Arsenal. But it is doubted over and over, like many old misconceptions. Now it is supposedly disproven by a computer program.


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Originally Posted by prm
For the experts out there, does barrel length for a given caliber change what powder will provide better speed?

the short answer is .....NO!

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Now it is supposedly disproven by a computer program.

Hardly. The program can only give suggestions as to what is going on behind the actual pressure traces.

I'll (try to) close with my original and unanswered question: how was it established that burn out comes near peak pressure? If someone will cite a reference by the real experts, I'll seek it out. My thermo professor did research in this field for the army, so the library there should contain many works on internal ballistics. Frankly, until this discussion, I'd never considered what happens near peak pressure, and I still haven't a definitive answer.

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My friend, I am not going to do your research for you. As you seem to be the lone voice in opposition to accepted theory, it is up to you to come up with counter evidence. As far as I've been able to determine (and I'll say it yet again), the end of effective burning is signalled by the pressure peak.


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I don't think there is that much difference in what everyone is saying. I think all he is stating is that not ALL burning is done at peak pressure, most, yes. Here is a picture of a 223 graph that includes bullet movement and velocity.

[Linked Image]

It definitely indicates there is still a small amount of burning past peak psi. Once the bullet has moved about three inches the volume of the combustion chamber (case + barrel behind bullet) has nearly doubled, yet the pressure has clearly not halved (actually only about a 20% drop) so something has to be happening. Clearly, this is only one example so take it for what it's worth. If the pressure graph showed the pressure dropping much more dramatically (as JB has stated he has obseved numerous times) that would certainly indicate a more complete burn at peak pressure. Time to go watch some gameday!

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Powder can blow out the pipe unburned-this I know from experience. I've loaded .375 -300 grain bullets in my Mark V Weatherby. I've packed those 378 weatherby jugs with 116 grains of 7828 behind the 300 grainers. I set the charge off with the shiity CCI primers. The last 5 grains did very little in the velocity gains. Theoretically if I could get a good burn possibly if I had a longer barrel. (not sure on that)There is alot of smoke and stink after that stuff blows out the pipe. I don't have fancy graphs but I can tell when the I get that big puff of smoke coming out the pipe that I'm not achieving a good burn. I didn't have Fed 215 primers but I know they are a hell of lot better igniters than the CCI in the big jugs. Kids don't do this at home. Detached retina stuff!!!!!


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CCI primers,eh. Now I know why H1000 and 7828 shot out smoke and smell of burning powder.
I am going to stick to Loads of 85-90% that are listed as max loads in the manuals.
Better groups for me. Not a rocket scientist, just a handloader.


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Originally Posted by prm
For the experts out there, does barrel length for a given caliber change what powder will provide better speed? In my case is there a benefit to using a somewhat faster, or slower powder, when using a shorter barrel (21" 338-06)? Not suggesting going outside the powders that typically work with a given caliber, just maybe leaning towards the faster end of those that would normally be considered for the 338-06. H4350 and RL17 are what I've been using, and they have worked well. Just curious if I should consider others (I have Varget and H4895 laying around).
................Whichever powders that are listed showing the top speeds from a 24" barreled 338-06, will also give you the best speeds from your 21" barreled 338-06. In other words, you load for that 21" barrel just as though you were loading for a 24" or a 26" barrel. The same is also holds true if loading for even the shorter tubes such as 20" or less.

Regardless of barrel length, whether from a 16.5" tube up to a 26" tube, the slower burners (where listed for a given cartridge showing greater speeds) generally across the board, will also give the better speeds in the shorter tubes.

RL 17 was primarily designed to give a velocity boost in the short mags. However, I read that it is doing well in other cartridges too.



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Whatever is going to burn gets consumed within a few inches of the case. Barrel length makes no difference for the burn time/amount.

The hot pressurized gases in the barrel are doing their work, pushing the bullet at an ever-increasing velocity. How the gases do the work is a function of the type of powder but his work comes to an abrupt halt when the bullet gets clear of the muzzle. The amount of work done by these gases is predicated by the length of the barrel. Whatever work they were still able to do when the bullet uncorks simply contributes to the muzzle blast.


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Thanks. I just want to get 2600 with a 225 Accubond or 2700ish with a 210 Scirocco II using a temperature stable powder. Should not be a problem. I've found that RL 17 works well in the 338-06. Nothing spectacular though. However, I caveat that with the fact that I don't have any 'official' load data to use as a guide so I am hesitant to push things. I have found that it gives the same velocity of H4350 but with one less grain of powder.

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prm, the plot reminds me of one I saw years ago in an NRA book. Having bullet motion and pressure on one time plot makes analysis much simpler, as you have pointed out. Being the plot of simulation results, it won't settle the question.

You mention the pressure did not halve. I'll note that if gas production ceases with peak pressure and if the volume has doubled after the peak, then the pressure should not be about 1/2 the peak, but instead should be around 40% of the peak.

When gases expand against a piston, they are doing work against it and transferring energy to it. This energy transfer causes pressure to drop faster than application of the basic gas law would suggest, which is the halving you mention. Instead, the pressure drops roughly according to this formula:

Code
 pressure ratio = (volume ratio) ^ k


where k is about 1.25 for propellant gases. For a doubling of volume, this represents about a 60% drop in pressure. Further, heat losses to the barrel and chamber accelerate this ideal pressure drop.

This is why I've been harping about the shape of all the pressure traces; they simply do not fall fast enough after peak pressure. Instead, they are well rounded near the peak pressure.

I found no suitable high pressure traces on the RSI page. The 7 STW plot there is interesting but suspect. The shape of the pressure curve (excluding the spike) demonstrates QL's limitations. Using the two available Nosler bullet weights and looking at several powders, I cannot get a pressure trace from QL that has the very slow fall off in pressure recorded nor the (astounding) indicated muzzle pressure. With the slowest ball powders, QL is predicting burning with the bullet well down the barrel.

Last edited by 2525; 09/26/09. Reason: simulation; RSI
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PRM, I had a 23" std. chamber 338/06, read LOTS of articles on the round by Finn Aaggard, Steve Timm, and others. The consensus showed IMR4320 to do very well.

Mine shot 2909 w/200s, 2790 w/215s and 2670 w/225s. IMR4320 shot top speeds and accuracy for me and I don't think much more could be gained and stuck with it, 1/2 MOA was norm in the Hart tube- a hvy sporter. RL17 might be just right, but I'd be 4350 would be better with the 250 and up class. If you want to try another powder and perhaps pick up a tad speed if needed, you might try 4320.

A 21" tube might cost you 30-50 fps IMO over the 23 I had which is moot. FWIW, I used WW brass, partial sized, std. primers. My throat was long, done for 250s, so my OAL was out there, maybe helping case capacity.

Your goal should be in reach safely IMO.

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In trying to learn when burn-out occurs relative to peak pressure, I've gone through the limited materials readily available here. The best reference at the local library is Corner's book. This was written in the UK just after the war to summarize the state of the art with regards to theoretical calculations. Digital calculations of the formulas used was just beginning and not greatly covered; at that time "computer" was generally a person's title, not a machine.

Results of simplified burning models were poor. These assumed burn-out occured near peak pressure, and they showed the fast fall off in pressure I've been noting, which does not match the rounded fall off his piezo data shows. Interesting are his comments on how burning in guns differs from that in pressure vessels due to erosive effects on the charge of the gas flow.

As the math models become more involved, the theoretical calculations begin to better match piezo data and the burn-out moves to after peak pressure. Confounding their calculations was the engraving of the projectiles into the barrel, a problem which confounds the calculations in sporting rifles to this day. Of note was that burn-out could in principle occur after the muzzle.

While quite interesting, Corner's work does not answer the question I raised. Corner was concerned with "guns" which are cannons and howitzers. He favorite example was a hefty AA gun. The propellants were not progressive; the engraving forces were relatively small; the loading densities were lower than in sporting rifles; and the chamber volumes were huge compared to the bore cross section.

It'll be months before I can get some time at the big engineering library down the road. I remember there is a book there by Krier with a section on the simulation of small arms circa 1970, and perhaps some answers might be found there. I do recall that bullet engraving was a problem on the models, but perhaps there will be some relevant experimental results.

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I have done a little testing in a 270, 30-06, 338 all of which performed best with slow burning powders in any given barrel length...I did this many years ago because the general concenses was that fast burning powders got more velocity in short barrels because the supposedly burned all the powder..For what its worth most of the early gun scribes promoted this therory in most of the magazines as I recall...

I tried it and I didn't get the same results they did, I was stringhaulter and gilflurted for my opinnion! smile smile smile..I don't know how some of this stuff gets started, its like some expert makes a wild claim and the whole world just excepts it because of who said it, but hey thats how we got our last president isn't it...:)

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