I see posts so frequently that warn that a particular powder can produce a dangerous "pressure spike" and folks asking for a load for a particular barrel length, that I though it might be useful to present a high level overview of what happens, pressure-wise in the barrel of a firearm when the trigger is pulled.
I pulled the graph below off the web from somewhere some time ago and modified it somewhat for my purposes. I can't provide specific credit for it, but want to start by saying it wasn't me. Thanks to its author whoever you are.
All powders, smokeless or blackpowder, "fast" or "slow", produce similar "Pressure Curves" and hence have similar "Internal Ballistics".
1. Upon firing, the pressure rises to a Peak Pressure very quickly (typically within 0.3-0.5 milliseconds) - that is less than 5 ten thousands of a second. During that time (rise to Peak Pressure) the bullet will have travelled at most 4-5 inches. You can see that point in the graph below as "A", "B" and "C" (for three different powders - explained later). The total time between firing and the bullet leaving the bore can be as little as 1 millisecond, but is rarely more than 4 milliseconds - that's 1/250 of a second; and
2. the pressure then drops very consistently as the bullet travels down the bore, in direct proportion to the increasing volume of the cylinder behind the bullet in the bore. Moments after the bullet leaves the bore, the pressure will be at zero (0). As you see in the chart, the bullet continues to accelerate even as the pressure is dropping - as long as the pressure is sufficient to overcome bore friction and bullet momentum, the bullet will move, if not accelerate. That's why longer barrels can exploit declining pressure more than shorter barrels - to a point.
Here's where the myths arise.
1. "You need a fast powder for short barrels to achieve a total burn" or "You need a long barrel with a slow powder to achieve a total burn". For barrels longer than about four (4) inches this is NOT TRUE.
The point where Peak Pressure occurs is the point where all the powder that will "burn" has been burnt. There is never 100% conversion of powders, and there are always "products of combustion" (e.g. soot). If the powder is not operating at its most efficient pressure there might indeed be "unburnt powder" among those products of combustion. The fact remains however, that at the point of Peak Pressure (A, B or C above) no more gasses are being produced, so with a fixed amount of gas, pressure will naturally drop as the bullet advances down the bore. That fire coming from the bore (muzzle flash) is real, but neither it, nor the soot that is produced is powder that didn't have a chance to burn - it's the products of combustion. So, unless the powder you are using hasn't yet reached Peak Pressure when the bullet exits the bore, you've achieved a "full burn". This is why powder that produce the highest Muzzle Velocity (MV) (at a given pressure) with long barrels will produce the highest MV with short barrels.
2. "This powder is notorious for producing dangerous pressure spikes". All powders produce a "Pressure Spike" - it's called the point of peak pressure. If you have more than one peak, it's due to a bore obstruction of "SEE" and that's a dangerous non-typical situation and Red Herring for this discussion. I think that what people mean when they say "This powder is notorious for producing dangerous pressure spikes" is that its peak pressure can rise quickly with small additions of powder, like with "fast" powders, which leads to:
"Powder Burn Rate"
Powders are designed to produce peak pressure in different ways - it might rise very quickly to a high peak with small amounts of powder ("Fast Powders"), or rise more slowly and require more powder to reach the same peak ("Slow Powders"), but at each end of the extreme it's still occurs in only a matter of fractions of a millisecond. In the chart above, the pressure curve for the fastest powder is "A", followed by "B" and "C". "A" would have used the least powder, followed by "B" and "C". You might ask: "Their Peak Pressures are all very close, so why not always use the least amount of the fastest powder?" The reason is that since every pound of powder contains roughly the same amount of energy, more powder used means more energy means higher MV and you might want that. Each firearm has a limit to the Peak Pressure it can withstand, so if you want a higher MV, you need to use a powder you can use more of without exceeding that Peak Pressure, i.e. a "Slower" Powder. The effect of using more powder (more energy) is captured in the chart by the area under curves "A", "B" and "C" - C is higher than B which is higher than A. You see this phenomenon in load chart (e.g. using more of H4350 I get the same Peak Pressure, but a higher MV than using less H4895). Here's a link to Hodgdon's "Powder Burn Rate Chart" which gives an approximation of relative burn rates: https://www.hodgdon.com/PDF/Burn Rates - 2014-2015.pdf
Please note that the graph of "Projectile Velocity" is a generic representation - each of powders "A", "B" and "C" would produce its own unique Projectile Velocity graph.
I hope this was useful.
I pulled the graph below off the web from somewhere some time ago and modified it somewhat for my purposes. I can't provide specific credit for it, but want to start by saying it wasn't me. Thanks to its author whoever you are.
All powders, smokeless or blackpowder, "fast" or "slow", produce similar "Pressure Curves" and hence have similar "Internal Ballistics".
1. Upon firing, the pressure rises to a Peak Pressure very quickly (typically within 0.3-0.5 milliseconds) - that is less than 5 ten thousands of a second. During that time (rise to Peak Pressure) the bullet will have travelled at most 4-5 inches. You can see that point in the graph below as "A", "B" and "C" (for three different powders - explained later). The total time between firing and the bullet leaving the bore can be as little as 1 millisecond, but is rarely more than 4 milliseconds - that's 1/250 of a second; and
2. the pressure then drops very consistently as the bullet travels down the bore, in direct proportion to the increasing volume of the cylinder behind the bullet in the bore. Moments after the bullet leaves the bore, the pressure will be at zero (0). As you see in the chart, the bullet continues to accelerate even as the pressure is dropping - as long as the pressure is sufficient to overcome bore friction and bullet momentum, the bullet will move, if not accelerate. That's why longer barrels can exploit declining pressure more than shorter barrels - to a point.
Here's where the myths arise.
1. "You need a fast powder for short barrels to achieve a total burn" or "You need a long barrel with a slow powder to achieve a total burn". For barrels longer than about four (4) inches this is NOT TRUE.
The point where Peak Pressure occurs is the point where all the powder that will "burn" has been burnt. There is never 100% conversion of powders, and there are always "products of combustion" (e.g. soot). If the powder is not operating at its most efficient pressure there might indeed be "unburnt powder" among those products of combustion. The fact remains however, that at the point of Peak Pressure (A, B or C above) no more gasses are being produced, so with a fixed amount of gas, pressure will naturally drop as the bullet advances down the bore. That fire coming from the bore (muzzle flash) is real, but neither it, nor the soot that is produced is powder that didn't have a chance to burn - it's the products of combustion. So, unless the powder you are using hasn't yet reached Peak Pressure when the bullet exits the bore, you've achieved a "full burn". This is why powder that produce the highest Muzzle Velocity (MV) (at a given pressure) with long barrels will produce the highest MV with short barrels.
2. "This powder is notorious for producing dangerous pressure spikes". All powders produce a "Pressure Spike" - it's called the point of peak pressure. If you have more than one peak, it's due to a bore obstruction of "SEE" and that's a dangerous non-typical situation and Red Herring for this discussion. I think that what people mean when they say "This powder is notorious for producing dangerous pressure spikes" is that its peak pressure can rise quickly with small additions of powder, like with "fast" powders, which leads to:
"Powder Burn Rate"
Powders are designed to produce peak pressure in different ways - it might rise very quickly to a high peak with small amounts of powder ("Fast Powders"), or rise more slowly and require more powder to reach the same peak ("Slow Powders"), but at each end of the extreme it's still occurs in only a matter of fractions of a millisecond. In the chart above, the pressure curve for the fastest powder is "A", followed by "B" and "C". "A" would have used the least powder, followed by "B" and "C". You might ask: "Their Peak Pressures are all very close, so why not always use the least amount of the fastest powder?" The reason is that since every pound of powder contains roughly the same amount of energy, more powder used means more energy means higher MV and you might want that. Each firearm has a limit to the Peak Pressure it can withstand, so if you want a higher MV, you need to use a powder you can use more of without exceeding that Peak Pressure, i.e. a "Slower" Powder. The effect of using more powder (more energy) is captured in the chart by the area under curves "A", "B" and "C" - C is higher than B which is higher than A. You see this phenomenon in load chart (e.g. using more of H4350 I get the same Peak Pressure, but a higher MV than using less H4895). Here's a link to Hodgdon's "Powder Burn Rate Chart" which gives an approximation of relative burn rates: https://www.hodgdon.com/PDF/Burn Rates - 2014-2015.pdf
Please note that the graph of "Projectile Velocity" is a generic representation - each of powders "A", "B" and "C" would produce its own unique Projectile Velocity graph.
I hope this was useful.
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