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#3330887 - 09/24/09 Barrel length vs. powder speed
prm Offline
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Registered: 10/27/08
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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).

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#3330932 - 09/24/09 Re: Barrel length vs. powder speed [Re: prm]
prm Offline
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I should add, I'm using 210 and 225gn bullets.

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#3331161 - 09/25/09 Re: Barrel length vs. powder speed [Re: prm]
2525 Offline
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Registered: 06/02/02
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No; you get the best speed by putting as much chemical energy (burned powder) behind the bullet as you can. To do this safely, you have to select a powder speed that prevents a case full of powder from completely burning until the bullet is far enough down the barrel that the space behind it can hold these gases at a safe pressure. Powder burn is complete (more or less) with the bullet less than a foot down the bore.

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#3331229 - 09/25/09 Re: Barrel length vs. powder speed [Re: prm]
Mule Deer Online   content
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Let's look at this step by step:

1) First, all the powder that's going to burn will burn within inches of the case mouth, much less than a foot. Fast-burning handgun powders will burn within less than an inch; the slowest burning rifle powders within 2-4 inches.

2) If the pressure is correct for the powder in question then almost all will burn. Let't call it 99%.

Some rifle powders are designed to burn best at 50,000 psi or so, others (usually the slower ones) at 60,000 psi. This is assuming we're talking about conventional bottle-necked cases designed for modern bolt-action rifles.

My own experiments, as well as those of many other people, indicate that the "best" (producing the highest muzzle velocity) powders for a given cartridge and bullet weight will be the best powders in any legal barrel length, which in the U.S. is 16".

The powder/bullet combination that will lose the least velocity in a short barrel is a slow powder with a heavy bullet.
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"Gunwriters, as you know, aren't as informed as their readers are and if it wasn't for the readers, there would be no need for writers..."--Shrapnel, May 2015

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#3331342 - 09/25/09 Re: Barrel length vs. powder speed [Re: Mule Deer]
prm Offline
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Thanks JB. Works for me. I see no reason to change from RL17 and H4350.

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#3332261 - 09/25/09 Re: Barrel length vs. powder speed [Re: Mule Deer]
2525 Offline
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I'm curious, John, how was it established that powder ceases to burn by about 4 inches of bullet travel?

I've read this elsewhere, but it's not obvious to me how it came to be known. To measure it physically is daunting. One not only needs to know pressure accurately, but bullet position in the barrel as well, and one also needs a handle on convective and radiative heat losses to the barrel and chamber.

It seems agreed that peak pressure occurs by about 4 inches of travel, but that does not necessarily imply gas production has ceased by then.

If established, this could explain why the QuickLoad simulator can underestimate peak pressure. For a 180 gn bullet in a .30-06, it estimates even a powder as fast as Re-15 is only about half burned by the point of peak pressure, and that a 90% burn comes about 9 inches down the barrel. It's mathematical estimations are hardly exact, but to track as well as it does the actual trends of internal ballistics over such a wide range of cartridges, I wouldn't expect it to be in gross error.

Regardless, I find nothing to contradict the claim that with legal length barrels, barrel length does not select optimum powder speed.

Karl

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#3332393 - 09/25/09 Re: Barrel length vs. powder speed [Re: 2525]
Mule Deer Online   content
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It has been extablished various authorities (Homer Powley among them) that the peak of the pressure curve is where all (or almost all) of the powder is burned. As I recall, this was established partly by cutting back barrels and seeing how much unburned powder was left at certain lengths.

Some people are under the impression that the peak is caused by the bullet fully engraving in the rifling, but that happens almost immediately in front of the chamber.
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"Gunwriters, as you know, aren't as informed as their readers are and if it wasn't for the readers, there would be no need for writers..."--Shrapnel, May 2015

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#3332424 - 09/25/09 Re: Barrel length vs. powder speed [Re: Mule Deer]
GunGeek Online   content
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Registered: 10/09/04
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Loc: NW Nevada
To speak of such things where rifle powders are concerned, gets a bit tough because thereís so many rifle cartridges out there, which brings about a lot of variables. With handguns, itís a bit easier because the sampling of both cartridges and powders is smaller. So letís take a magnum revolver cartridge, say the more popular ones, such as .357, .41 & .44 magnums. With these powders, generally speaking itís the slower powders that give you the highest velocities (H110/296 etc). This is true regardless of barrel length.

With rifles, you canít make such a statement, because not always is it the slowest powder that gives you the highest velocities. But suffice to say, whatever powder gives you the best velocity in a 24Ē barrel, is probably going to be the one that gives you the highest velocity in a 16Ē barrel.

However, whether weíre talking revolver or rifle, that increased velocity in short barrels often comes at the expense of greatly increased noise and muzzle blast; which is not always desirable for this shooter.
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#3332449 - 09/25/09 Re: Barrel length vs. powder speed [Re: Mule Deer]
2525 Offline
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Registered: 06/02/02
Posts: 534
Loc: U.S.A.
The shape of pressure traces clearly indicates that burning continues past peak pressure, so perhaps barrel chopping experiments missed energy released inside the prodigious muzzle blast. Consider this trace from RSI's site:


If gas production had ceased by the point of peak pressure, the fall off in pressure would be similar to that of an adiabatic expansion. Such curves have sharp fall offs, yet the traces show a very broad and rounded fall off.

This would have been noticed even in the 1920s, so I remain at a loss to explain the information at hand.

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#3332484 - 09/25/09 Re: Barrel length vs. powder speed [Re: 2525]
2525 Offline
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Registered: 06/02/02
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For grins, I had QuickLoad simulate a burn of enough Varget to produce about 44 ksi peak pressure. It's estimated time trace is remarkably similar in shape to that of the RSI traces. It figures a fair amount (15%) of powder remains to be burned as late as 0.7 msec. Both QL and the RSI traces show points of inflection (in the pressure traces) at about 0.7 msec, indicating the bullet is well out pacing the production of gases, to the point that the curve is starting to look like a simple expansion. At this point, QL estimates the bullet has moved about 6 inches with powder to go.

I wish I had the time and money to run proper experiments on this stuff...


Edited by 2525 (09/25/09)

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#3332565 - 09/25/09 Re: Barrel length vs. powder speed [Re: 2525]
RockyRaab Offline
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Registered: 05/23/03
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I don't think I'd rely overmuch on QuickLoad. As brilliant as the program and its creator are, it is still software. That means that it will spit out pretty much what the assumptions and formulae you feed in create. Actual pressure traces are more trustworthy because they report what is actually happening.

BTW, I disagree with what 2525 says. If all the powder that is going to burn is ignited more or less simultaneously, then it will burn out more or less simultaneously. That occurs at the pressure peak; again, more or less.

Some powder never does burn. In a bottleneck case, it is probable that a "plug" of powder gets compressed and rammed into the back of the bullet at primer firing. That dense plug is too compact for primer gasses to penetrate and cause ignition. Only the rearmost surface layer can and will burn. Any residual burning from such powder kernels might explain why the post-peak curve is shallower than we might expect. Note, however, that with the pressure dropping, any post-peak burning is much less efficient than otherwise simply because the pressure is below the optimum for that powder. That's why we can say that the peak marks the end of effective burning.

In any event, by timing the pressure curve, it is quite simple to know where the bullet was at any given point on the curve.
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#3332720 - 09/25/09 Re: Barrel length vs. powder speed [Re: RockyRaab]
2525 Offline
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Registered: 06/02/02
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Loc: U.S.A.
I won't rely on the QuickLoad outputs as gospel, but the math behind it is beyond mere assumptions; the formulas it uses are based on empirical research. Having coded software simulations of physical processes myself, I'm aware they can be embarrassingly inaccurate, but I'm also aware they can provide insights that empirical research can be slower to highlight.

It wasn't until I compared QL's traces to that from RSI that it dawned on me that RSI's traces showed it's not possible for the burn to have been completed near peak pressure, as I outlined above. Once the burn is complete, the pressure trace will be concave (as viewed from "above" the curve). Instead, it is convex over half the trace.

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#3332846 - 09/25/09 Re: Barrel length vs. powder speed [Re: 2525]
ricksmith Offline
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Registered: 02/15/04
Posts: 2081
Loc: sc
How would the pressure show the drop with the bullet still in the barrel? Once pass peak pressure there is a slow decline due to the lack of more usable pressure being added by burning powder and by the bullet progressing down the barrel.Rick.

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#3332866 - 09/25/09 Re: Barrel length vs. powder speed [Re: 2525]
Mule Deer Online   content
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Registered: 07/24/01
Posts: 36643
Loc: Banana Belt, Montana
No, the peak of the pressure curve is caused by the powder burning and creating MORE gas. Once the amount of gas is stable (because the powder is burned) then the pressure curve starts downward. The gas is expanding, because it's hot, but no more is being created by burning powder. The pressure curve drops because the volume of the bore increases as the bullet travels further along.
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"Gunwriters, as you know, aren't as informed as their readers are and if it wasn't for the readers, there would be no need for writers..."--Shrapnel, May 2015

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#3332968 - 09/25/09 Re: Barrel length vs. powder speed [Re: Mule Deer]
2525 Offline
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Registered: 06/02/02
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Loc: U.S.A.
No, that the curve starts down only indicates that the bullet is creating volume faster than this volume can be filled by the burning of the remaining powder. It does not prove gas production has ended.

Now, if there is no more gas (and energy) being released, then the pressure vs. volume curve will closely follow a shape of the form P*(V^k)=c. Plotted against volume, this curve is strongly concave upwards. We're looking at P plotted against time instead. Here, the bullet's acceleration will stretch out the curve, but I'm confident (which doesn't mean I'm correct!) it cannot invert the shape of the curve.

The RSI data shows a convex curve after the pressure peak.

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#3333176 - 09/25/09 Re: Barrel length vs. powder speed [Re: 2525]
RockyRaab Offline
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Registered: 05/23/03
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I am most skeptical that the bullet could possibly create more volume by its movement than the powder can fill with gas. I think that's the flaw in your theory.

Any convexity in the curve after the absolute peak is due to the "tailoff" of residual propellant, but that is a minor and short-lived event, as evidenced by the faster (and concave shape) shortly after the peak.

The bottom line is that pressure builds as long as there is deflagration. By definition then, when the pressure ceases to build, burning has (essentially) completed.
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#3333203 - 09/25/09 Re: Barrel length vs. powder speed [Re: RockyRaab]
FTR_Shooter Offline
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Registered: 01/31/09
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Loc: The Lone Star State
I initially thought the same, but I'm willing to bet that 2525 was trying to say that the burning of the remaining powder was unable to sustain the same pressure as the volume expanded due to the now-moving bullet.

On the other hand, I think he may be incorrect in thinking the bullet is no longer accelerating once the powder has burned completely (or as completely as it's going to burn; there is usually up to 15% unbunred powder.) The bullet will continue to accelerate as long as the pressure behind it is greater than the resistance of the bullet against the barrel. The Delta Vee will be reducing as the pressure behind the bullet drops, but it's not going to be negative.

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#3333343 - 09/25/09 Re: Barrel length vs. powder speed [Re: FTR_Shooter]
2525 Offline
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Registered: 06/02/02
Posts: 534
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Rocky, I believe that what powder remains can't fill the void fast enough. The grains are smaller and present much less surface area, and this limits gas production, and once the pressure curve begins to fall off, the burning rate falls rapidly, compounding the situation.

What's this "tail-off," and if it is a minor and short lived event, then why does the curve remain convex for about .3 ms, which is on the order of the pressure rise time recorded?

FTR, I didn't claim the bullet stops accelerating; it most certainly keeps accelerating! The acceleration is why the pressure-time trace is flatter than the pressure-volume trace. It's because the acceleration is due to the pressure that I'm confident switching from a p-v curve (which I'm certain must be concave upwards) to a p-t curve will still leave you with a curve that must be concave upward (after the burn is done). Real curves are convex, so I believe burning continues--as QL predicts.

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#3333419 - 09/25/09 Re: Barrel length vs. powder speed [Re: 2525]
2525 Offline
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Registered: 06/02/02
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I've taken this discussion a bit off the topic, eh? grin

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#3333485 - 09/25/09 Re: Barrel length vs. powder speed [Re: 2525]
prm Offline
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Registered: 10/27/08
Posts: 5331
Nope, it's interesting. I'll just rephrase my question to what powder psi trace will provide the largest volume under the curve for a given bullet and barrel length, without exceeding max psi?

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#3333637 - 09/25/09 Re: Barrel length vs. powder speed [Re: prm]
1234567 Offline
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Registered: 11/26/07
Posts: 4320
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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#3333693 - 09/26/09 Re: Barrel length vs. powder speed [Re: 2525]
Mule Deer Online   content
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Registered: 07/24/01
Posts: 36643
Loc: Banana Belt, Montana
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.


Edited by Mule Deer (09/26/09)
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#3333739 - 09/26/09 Re: Barrel length vs. powder speed [Re: Mule Deer]
RockyRaab Offline
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Registered: 05/23/03
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Loc: Ogden, Utah
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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#3333789 - 09/26/09 Re: Barrel length vs. powder speed [Re: prm]
Mule Deer Online   content
Campfire Oracle

Registered: 07/24/01
Posts: 36643
Loc: Banana Belt, Montana
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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