I was answering a question on another tread and the topic of slow verses fast powders verses barrel length came up. One of the posters on the other thread said “I believe the slowest powders will give the highest velocity in all barrels. But in short barrels they will gain less (percentage wise) compared to the faster powders.†This statement was referring to 38s. I guess I really never gave this topic that much though. Now I’m wondering what the facts really are. I’m referring to handguns not rifles. What determines which powder gives you the fastest bullet speed (FPS) for a given cartridge and how does barrel length effect this?
slow verses fast powders verses barrel length
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Zak Smith
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As the powder is ignited and burns, the pressure by time curve reaches a maximum pressure region rather quickly, and then trails off. As long as the barrel is long enough that this max pressure regime can be attained, the slower powder which gives you max velocity in the longer barrel will also give you the max velocity in the shorter barrel. If you cut the barrel down shorter than that, the slow powder load will not reach its max burn rate, and you'd need to step to a faster burning powder in order for that one to reach its max burn rate.
In rifles, the max pressure generally happens within the first 6" of bullet travel.
If you want to get an intuitive understanding of this, go buy a copy of QuickLoad from NECO software and play with it for a while.
ETA:
To answer the second question: in general, the slowest burn rate powder which you can both fit enough in the case and it can reach the maximum pressure with the bullet in the barrel will produce the maximum velocity. The goal is to have the maximum area under the pressure/distance curve, since that is directly related to the bullet's energy and thus velocity by the bore area.
In rifles, the max pressure generally happens within the first 6" of bullet travel.
If you want to get an intuitive understanding of this, go buy a copy of QuickLoad from NECO software and play with it for a while.
ETA:
To answer the second question: in general, the slowest burn rate powder which you can both fit enough in the case and it can reach the maximum pressure with the bullet in the barrel will produce the maximum velocity. The goal is to have the maximum area under the pressure/distance curve, since that is directly related to the bullet's energy and thus velocity by the bore area.
rbernie
Contributing Member
Zak covered this pretty bloody thoroughly, but I'd like to add a few ancillary thots. There are many factors other than barrel length and powder speed (oxidation rate) that can affect the pressure curve.
For example, heavy neck tension or a crimp can dramatically increase PEAK pressures (and velocity). This effect can, if applied intelligently and judiciously, allow the use of 'slower' powders than might otherwise be the norm - effectively 'lengthening the barrel' by artifically constraining the gases to a smaller volume for a longer period of time. Put a light crimp a caseful of 2400 in your 357Mag and then try the same load with a heavy crimp, and you'll both see and be able to measure on a chrono the difference in velocity. (Try that same experiement with a max load of a faster powder, and you may get to wear parts of the receiver in your forehead as you create a pressure spike that's too much for the action to handle..)
Along the same line of thought, the shape of the case can impact flame front propagation and therefore cause slower-than-expected or faster-than-expected pressure rises. In straightwall pistol cases, this isn't really an issue but it is certainly a factor in bottleneck rifle chamberings.
Another factor is the primers used. Some primers create a larger flame nucleus than others, causing a delta in the pressure curve based upon the amount of powder that is initially ignited. This effect is usually pretty subtle, but I've seen instances (mostly with ball powders, mostly in rifles) where changing the primers created a +-10% delta in velocity with no change in accuracy.
None of this is meant to imply that some basic common sense (as Zak detailed) with regard to burn rates and barrel lengths doesn't apply. I'm simply trying to point out that there are additional variables that let you wiggle around a bit within the basic parameters.
Art Eatman
Moderator In Memoriam
To add a bit to Zak's comments:
If you look at the velocities for various powders as given in a reloading manual, you'll see that faster powders reach their peak pressures earlier than the slower ones.
You'll also see that as barrel length is reduced (from, say, 6" to 2"), there is a velocity loss with any powder. But, the loss is a bit greater per inch for the slower powders.
What happens with faster powders in the shorter barrels is that you get less unburned powder spewed out, and there is a bit less muzzle flash.
I first got into this with a couple of Blackhawks, many years ago (late 1960s). Max loads of 2400 in the shorter barrel (5"?) lit up the world if shot at night. With a longer, 7" (7-1/2"?) barrel, the flash was less. With max loads of Bullseye, the flash was even less from either barrel.
FWIW, Art
If you look at the velocities for various powders as given in a reloading manual, you'll see that faster powders reach their peak pressures earlier than the slower ones.
You'll also see that as barrel length is reduced (from, say, 6" to 2"), there is a velocity loss with any powder. But, the loss is a bit greater per inch for the slower powders.
What happens with faster powders in the shorter barrels is that you get less unburned powder spewed out, and there is a bit less muzzle flash.
I first got into this with a couple of Blackhawks, many years ago (late 1960s). Max loads of 2400 in the shorter barrel (5"?) lit up the world if shot at night. With a longer, 7" (7-1/2"?) barrel, the flash was less. With max loads of Bullseye, the flash was even less from either barrel.
FWIW, Art
fecmech
Member
There has always been a discussion about barrel lengths and powder burn rates but simply put whatever is fastest out of a 6" tube is fastest out of a 2 " tube. I'm not talking efficiency, muzzel blast, muzzel flash or anything else, just plain fps. The only time a fast powder would beat a slow powder is if it was equal to or faster than the slow powder in the 6" which would be very unusual.
Zak Smith
Member
To add to what rbernie said, another one of those additional factors in rifles especially is the freebore or distance the bullet jumps before it engages the rifling. As the throat erodes due, one effect you see is that the exact same load you were shooting before will now produce less muzzle velocity. For example, a 50BMG load that used to shoot over 2600fps, might now not be able to break 2500fps. Throat erosion generally isn't an issue in pistols, but freebore distance can be.
-z
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"but simply put whatever is fastest out of a 6" tube is fastest out of a 2 " tube"
After reading the above posts, this doesnt appear to be the case, unless I am misunderstaning something. If this slower powder did not reach its max pressure in the 2" barrel, then wouldnt the faster powder yield a higher velocity, since it would generate a higher pressure in the 2" barrel? The way I am understanding it, is both the fast and slow powder would be burning the full length of the 2" barrel, and the faster powder would rise to a higher pressure, creating more fps. Please correct me if I am wrong
After reading the above posts, this doesnt appear to be the case, unless I am misunderstaning something. If this slower powder did not reach its max pressure in the 2" barrel, then wouldnt the faster powder yield a higher velocity, since it would generate a higher pressure in the 2" barrel? The way I am understanding it, is both the fast and slow powder would be burning the full length of the 2" barrel, and the faster powder would rise to a higher pressure, creating more fps. Please correct me if I am wrong
Zak Smith
Member
candt,
The issue is whether or not each powder will its its max pressure -- the pressure it would reach in a sufficiently long -- in the shorter barrel.
For typical handgun powders, I'm almost certain this occurs within the first 2". Heck, it's true for many rifle loads!
The issue is whether or not each powder will its its max pressure -- the pressure it would reach in a sufficiently long -- in the shorter barrel.
For typical handgun powders, I'm almost certain this occurs within the first 2". Heck, it's true for many rifle loads!
This is a thread I posted a couple years ago where I chronoed a few different loads out of various barrel lengths in both .357 and 9mm. The thread contains very little theory but it does contain some actual chrono data.
http://www.thehighroad.org/showthread.php?t=45283&highlight=chrono
http://www.thehighroad.org/showthread.php?t=45283&highlight=chrono
Zak Smith
Member
Great data 444, and it teaches us that general theory can only go so far. There can be exceptions to this rule when there are two powders close in speed but the "trail off" rates are different, but note that the difference between load #1 and #3 is still only 35fps.
RugerOldArmy
Member
A lot of good points have been made, and in general, I think the premise is true. However there are exceptions, and many good exceptions were listed.
One that was not is the 'amount of shoulder'. If you look at Speer 13, on page 376, 35 Whelen is discussed. Speer notes: 'Powders of moderate burning rates provide the best performance in the Whelen. Because the case has a minimum amount of shoulder, the slow burning propellants often give less velocity, even with heavy bullets."
One that was not is the 'amount of shoulder'. If you look at Speer 13, on page 376, 35 Whelen is discussed. Speer notes: 'Powders of moderate burning rates provide the best performance in the Whelen. Because the case has a minimum amount of shoulder, the slow burning propellants often give less velocity, even with heavy bullets."
This data is for 38 Special Gold Dot 110 Gr. Bullets, but it will show you something about bullet speeds and burn rates.
1- Fast burn rate - 40 – Slower burn rate
Burn rate- Powder- MAX load fps
22 Power Pistol 1074
31 Viht 3n37 1071
29 AA#5 1043
8 titegroup 1002
21 H Universal 998
6 700-X 997
7 Bullseye 990
19 231 971
9 American Select 887
These were shot in a 6 inch barrel. As you can see fps and burn rate don’t seem to have a set order. These are listed in order of FPS.
This is what made me start this thread. I wonder if the order would change in a 2 inch barrel?
1- Fast burn rate - 40 – Slower burn rate
Burn rate- Powder- MAX load fps
22 Power Pistol 1074
31 Viht 3n37 1071
29 AA#5 1043
8 titegroup 1002
21 H Universal 998
6 700-X 997
7 Bullseye 990
19 231 971
9 American Select 887
These were shot in a 6 inch barrel. As you can see fps and burn rate don’t seem to have a set order. These are listed in order of FPS.
This is what made me start this thread. I wonder if the order would change in a 2 inch barrel?
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