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#3330887 - 09/24/09 03:22 PM Barrel length vs. powder speed
prm Offline
Campfire Guide

Registered: 10/27/08
Posts: 4782
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 03:44 PM Re: Barrel length vs. powder speed [Re: prm]
prm Offline
Campfire Guide

Registered: 10/27/08
Posts: 4782
I should add, I'm using 210 and 225gn bullets.

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#3331161 - 09/24/09 05:19 PM Re: Barrel length vs. powder speed [Re: prm]
2525 Offline
Campfire Regular

Registered: 06/02/02
Posts: 520
Loc: U.S.A.
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 - 09/24/09 05:45 PM Re: Barrel length vs. powder speed [Re: prm]
Mule Deer Online   content
Campfire Oracle

Registered: 07/24/01
Posts: 33184
Loc: Banana Belt, Montana
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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#3331342 - 09/24/09 06:23 PM Re: Barrel length vs. powder speed [Re: Mule Deer]
prm Offline
Campfire Guide

Registered: 10/27/08
Posts: 4782
Thanks JB. Works for me. I see no reason to change from RL17 and H4350.

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#3332261 - 09/25/09 07:12 AM Re: Barrel length vs. powder speed [Re: Mule Deer]
2525 Offline
Campfire Regular

Registered: 06/02/02
Posts: 520
Loc: U.S.A.
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 08:15 AM Re: Barrel length vs. powder speed [Re: 2525]
Mule Deer Online   content
Campfire Oracle

Registered: 07/24/01
Posts: 33184
Loc: Banana Belt, Montana
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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The ultimate concern of a rifle loony is rifle trivia. And why not? What else is as distracting from the really important concerns of everyday life?

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#3332424 - 09/25/09 08:31 AM Re: Barrel length vs. powder speed [Re: Mule Deer]
GunGeek Offline
Campfire Kahuna

Registered: 10/08/04
Posts: 16288
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 08:41 AM Re: Barrel length vs. powder speed [Re: Mule Deer]
2525 Offline
Campfire Regular

Registered: 06/02/02
Posts: 520
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 08:55 AM Re: Barrel length vs. powder speed [Re: 2525]
2525 Offline
Campfire Regular

Registered: 06/02/02
Posts: 520
Loc: U.S.A.
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 09:11 AM)

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