Pressure differnces and powder burn rates

Adamantium · February 7, 2002, 02:22 PM

I've just gotten started on reloading shotshells, and I've been looking at the pressure differences between the same loads using different powders. The two powders I've used so far is Red Dot and Green Dot, Red Dot is the faster burning powder and generates more presure between the two using the same dram equiv. And it seems that Red Dot is more recommended for lighter loads from what I've read, while Green is used most for medium power loads.

I've got an idea for the reason for that and I'm wondering if you guys would tell me whether I'm thinking about this correctly or out in left feild. The faster burning powder is recommended for lighter loads because less powder will generate more pressure and cylce auto's. While using a slower burning powder, the round of ammo itself would shoot fine, but it wouldn't generate enough pressure and the gun would have a failure to eject. And the opposite, using a heavy load of faster burning powder would slam the bolt back with more than enough force than needed and would ruin the gun over time. That being said I am assuming the is a ideal "Presure Window" for all different shotshells and cartridges that will be used in automatic guns. And that guns like pump and bolt action guns have a wider range of choises in powder uses because they are manually cylced, obviously staying within safe reloading data.

Am I correct in thinking this?

Sincerely,
Adam

**Mike Irwin** · February 7, 2002, 04:05 PM

I think you'll find that the "pressure window" is a lot wider than you think it is, and that for a lot of semi-autos there are a couple of factors at work in getting the system to cycle.

The reason you use faster burning powders for light loads, and slower burning powders for heavy loads, is because smokeless powder is progressive burning -- meaning that as pressure builds in the shell, the powder burns faster and more efficiently, producing more pressure, which makes the powder burn faster and more efficiently. It's a loop.

In a light load, there isn't enough resistance for a slower-burning powder to develop the pressure it needs to burn efficiently -- essentially the loop is interrupted.

In the case of a fast powder with a heavy load, things become TOO efficient, and pressures can run right past the safe zone and into the BOOM zone really quickly.

Now, this leads to one possible factor that makes both types of powders work in semi-autos.

The faster burning powder develops higher pressure, but the pressure curve is usually VERY short, but the pressure is high enough to operate the action on the shotgun.

With a slower burning powder, yes, the pressure is lower, but because the slower burning powder takes longer to burn, the pressure curve is usually MUCH longer. It's this long pressure curve that allows the action to operate even at lower peak pressures.

Now, generally, and in the older shotguns (before self-adjusting gas systems), you would have two gas port settings, one labeled H (high power, small gas port) and one labeled L (low power, large gas port).

In my experimenting with my father's Remington 58 Sportsmen, I've found that the light shot loads using fast burning powder require the L setting, while heavier loads with the slower power require the H setting.

I'm pretty certain that's because of the relative length of time of the pressure curve, the faster powder gives a shorter push, so you need a bigger gas port to overcome the inertia of the action, while the slower powder gives a more sustained (but lowerer pressure, push so you need a smaller gas port.

I hope that some of this makes sense.

Adamantium · February 7, 2002, 10:46 PM

Thanks, thats what I wanted to know.

I've just got one question about what you said about efficientcy. Is that the reason that to get the same velocity with a slightly slower burning powder the recipe calls for a few more grains of powder? I.E. if 2 3/4 dram equiv. in Red Dot was 18.5 gr then it would be something like 20 gr. Green Dot.

Sincerely,
Adam

Edit: Forgot to finish the last part of my last sentance, just ended midway through it.

**Mike Irwin** · February 7, 2002, 10:57 PM

Well, I'd think that that would be the primary reason, but I'm not 100% certain if that's the only reason.

Poodleshooter · February 8, 2002, 05:14 PM

Another thing that you'll find in shotshell reloading is that available space is a big concern in deciding on a powder. The faster powders are usually very compact, whereas the slower powders take up more space, in addition to being weighty. This is why the wad columns get shorter for heavy field loads (which need slow powders) as opposed to light target loads.
Some hints for developing shotshell loads: if your load fits and crimps well, but you have lots of unburned granules in the barrel with a given shot weight, hull, and wad, you need to switch to a faster powder for that combo. A burned out or holed crimp area, or holes in a relatively new hull will often indicate a powder that is a little too fast.
BTW, I really like Green Dot. It has a reputation for shooting tight patterns, and is easier on my hulls than Clays or Red Dot.

February 7, 2002, 02:22 PM	#1
Adamantium Senior Member Join Date: June 7, 1999 Location: El Paso, TX Posts: 1,021	Pressure differnces and powder burn rates I've just gotten started on reloading shotshells, and I've been looking at the pressure differences between the same loads using different powders. The two powders I've used so far is Red Dot and Green Dot, Red Dot is the faster burning powder and generates more presure between the two using the same dram equiv. And it seems that Red Dot is more recommended for lighter loads from what I've read, while Green is used most for medium power loads. I've got an idea for the reason for that and I'm wondering if you guys would tell me whether I'm thinking about this correctly or out in left feild. The faster burning powder is recommended for lighter loads because less powder will generate more pressure and cylce auto's. While using a slower burning powder, the round of ammo itself would shoot fine, but it wouldn't generate enough pressure and the gun would have a failure to eject. And the opposite, using a heavy load of faster burning powder would slam the bolt back with more than enough force than needed and would ruin the gun over time. That being said I am assuming the is a ideal "Presure Window" for all different shotshells and cartridges that will be used in automatic guns. And that guns like pump and bolt action guns have a wider range of choises in powder uses because they are manually cylced, obviously staying within safe reloading data. Am I correct in thinking this? Sincerely, Adam

February 7, 2002, 04:05 PM	#2
Mike Irwin Staff Join Date: April 14, 2000 Location: Northern Virginia Posts: 41,642	I think you'll find that the "pressure window" is a lot wider than you think it is, and that for a lot of semi-autos there are a couple of factors at work in getting the system to cycle. The reason you use faster burning powders for light loads, and slower burning powders for heavy loads, is because smokeless powder is progressive burning -- meaning that as pressure builds in the shell, the powder burns faster and more efficiently, producing more pressure, which makes the powder burn faster and more efficiently. It's a loop. In a light load, there isn't enough resistance for a slower-burning powder to develop the pressure it needs to burn efficiently -- essentially the loop is interrupted. In the case of a fast powder with a heavy load, things become TOO efficient, and pressures can run right past the safe zone and into the BOOM zone really quickly. Now, this leads to one possible factor that makes both types of powders work in semi-autos. The faster burning powder develops higher pressure, but the pressure curve is usually VERY short, but the pressure is high enough to operate the action on the shotgun. With a slower burning powder, yes, the pressure is lower, but because the slower burning powder takes longer to burn, the pressure curve is usually MUCH longer. It's this long pressure curve that allows the action to operate even at lower peak pressures. Now, generally, and in the older shotguns (before self-adjusting gas systems), you would have two gas port settings, one labeled H (high power, small gas port) and one labeled L (low power, large gas port). In my experimenting with my father's Remington 58 Sportsmen, I've found that the light shot loads using fast burning powder require the L setting, while heavier loads with the slower power require the H setting. I'm pretty certain that's because of the relative length of time of the pressure curve, the faster powder gives a shorter push, so you need a bigger gas port to overcome the inertia of the action, while the slower powder gives a more sustained (but lowerer pressure, push so you need a smaller gas port. I hope that some of this makes sense. __________________ "The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza Baby Jesus cries when the fat redneck doesn't have military-grade firepower.

February 7, 2002, 10:46 PM	#3
Adamantium Senior Member Join Date: June 7, 1999 Location: El Paso, TX Posts: 1,021	Thanks, thats what I wanted to know. I've just got one question about what you said about efficientcy. Is that the reason that to get the same velocity with a slightly slower burning powder the recipe calls for a few more grains of powder? I.E. if 2 3/4 dram equiv. in Red Dot was 18.5 gr then it would be something like 20 gr. Green Dot. Sincerely, Adam Edit: Forgot to finish the last part of my last sentance, just ended midway through it. Last edited by Adamantium; February 8, 2002 at 02:41 AM.

February 7, 2002, 10:57 PM	#4
Mike Irwin Staff Join Date: April 14, 2000 Location: Northern Virginia Posts: 41,642	Well, I'd think that that would be the primary reason, but I'm not 100% certain if that's the only reason. __________________ "The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza Baby Jesus cries when the fat redneck doesn't have military-grade firepower.

February 8, 2002, 05:14 PM	#5
Poodleshooter Senior Member Join Date: August 8, 2000 Location: Floating down the James River in VA Posts: 2,599	Another thing that you'll find in shotshell reloading is that available space is a big concern in deciding on a powder. The faster powders are usually very compact, whereas the slower powders take up more space, in addition to being weighty. This is why the wad columns get shorter for heavy field loads (which need slow powders) as opposed to light target loads. Some hints for developing shotshell loads: if your load fits and crimps well, but you have lots of unburned granules in the barrel with a given shot weight, hull, and wad, you need to switch to a faster powder for that combo. A burned out or holed crimp area, or holes in a relatively new hull will often indicate a powder that is a little too fast. BTW, I really like Green Dot. It has a reputation for shooting tight patterns, and is easier on my hulls than Clays or Red Dot.