I'm trying to understand what is needed from the powder for the most consistent velocity from handgun loads. What I mean by consistency in muzzle velocity is quantified by multiple samples of 5-shot ES < ~20fps and SD < ~10fps. I know this kind of consistency may have "no practical use," but its use for me is to help me understand how the characteristics of propellant affects the end result.
I believe it is common practice to "ladder" loads to discover consistency and accuracy of certain combinations of load characteristics during load development. Powder type and charge mass are just some of the variables we ladder, but they are the ones that are the topic of this thread. We've all seen some loads that have a very tight extreme spread and small standard deviation, and other loads that are wildly inconsistent. Consistency in velocity is not solely, maybe not even primarily a function of powder, but it is a factor. Because other factors related to the gun, chamber, barrel, the brass, sizing, crimp, the primer, bullets, and so on can be identified by isolating those variables, I want to discuss and understand how different powder qualities affect consistency.
One factor seems to be "ignitability" -- that is how easily the flakes or balls of powder are ignited. I understand that single base powders are made mostly of nitrocellulose, while double-base powders substitute nitroglycerin for up to 42% of the mass, with most of the remainder being made up of nitrocellulose. With only those ingredients, we would have only "fast burning" powders. Deterrents are added to cause the powders to burn and gasify more slowly, creating pressure more gradually. This allows a greater amount of energy to be delivered within a maximum peak pressure because the energy is delivered over a longer time. But deterrents could make the powder harder to ignite. We may need to use large size primers, "magnum" primers, or rifle primers to get the best combustion of these powders. I realize the distinctions of those different primers is more than just greater ignition -- primers are discussed in other threads. Some obvious examples of hard to ignite handgun powders are H110, IMR4227, and Lil'Gun. But ignitability does not seem to be a binary feature with options for "easy" and "hard." There seems to be a wide range if ignitability among different propellants and the volume and compression of the powder in the case seems to affect the ignitability as well as the pressure during combustion.
Ignitability does not seem to be as much of a problem in rifles with their long barrels. The long barrel gives the powder charge plenty of time to fully ignite when it is under pressure before the bullet base exits the muzzle and the pressure drops rapidly. Because of this, we can use much slower rifle powders. We know if we loaded a handgun cartridge with rifle powder and a bullet and ignited it in the chamber of a short barrel that a large portion of the charge would not complete combustion before the bullet base exited. The pressure would rapidly drop and some of the powder might not even burn at all. It seems that large volumes of slow powders in short barrels create the circumstances where lower "ignitability" creates inconsistent velocities.
I suppose a certain volume of powder -- the volume that a given case restricts us to -- contains so much energy from the NC and NG. The more of that volume that is taken up by deterrent, the less total energy it has. As we go to more and more heavily deterred, slower powders, we need larger and larger cases to hold the greater volume of deterrent without reducing the volume we're filling with NC and NG. So given a fixed case capacity for a particular cartridge, we can only go so slow in burn rate before we start losing energy that could be released within the pressure limit. As we approach this limit, a greater and greater portion of the charge is still combusting when the bullet base exits the muzzle. My conjecture, I don't know if its true, but I think that the smaller the portion of the total charge that finishes combustion before base exit, the greater the opportunity for random variability in the resulting pressure/time curve exists. The greatest consistency would result from a condition where 100% of combustion always completes before base exit. This would cost the opportunity of higher pressures at the time of base exit, and therefore lower overall velocity, but it would best assure consistency in the velocity.
Another factor in the consistency of velocity for handgun cartridges is "position sensitivity." On the other end of the spectrum from large, slow charges we have tiny charges of fast powders. These kinds of loads appear to have what it takes to deliver excellent consistency in velocity unless the case has excess volume. With small charges in large cases like .357 Magnum or .45 Long Colt, we could see differences in the resulting velocity when there is inconsistency with where the powder sits in the case -- against the primer, piled up behind the bullet, or spread out along the length of the case. Again, with short the barrels, it seems more likely we will see differences in the result based on differences in how combustion began. Long rifle barrels seem to even the results out.
Hodgdon advertises its "Titegroup" powder as having characteristics that make it consistent even with small charges in large cases. They claim it has less position sensitivity. They also claim it is more consistent with a variety of differences in primers used. I'm not sure what it is about Titegroup that makes this possible, but it seems to have something to do with "ignitability." Whatever it is, is it restricted to fast-burning powders? Is it restricted to low-volume/low-mass powder charges? Is it a characteristic derived from the portion of nitroglycerin used? I know it is not a result of high NG % alone. Lil'Gun is infamous for having a high NG content, but it is also heavily-deterred, ignites poorly with standard primers, and shows a lot of inconsistency especially in shorter handgun barrels. That brings up the question: because Titegroup is a double-base powder, it seems evident that some double-based powders can produce consistent velocities in handguns. Can single-base powders produce consistent velocities in handguns? The only one I have is IMR4227 and it doesn't seem consistent at all, but it is also exceptionally slow for a handgun powder.
Is there a limit to powder burn rate and barrel length where consistency in muzzle velocity drops away? Can we begin to quantify this limit?
To recap, I think variability in how combustion begins due to the ignitability of the propellent, variability in how combustion proceeds due to the position of the powder in the case, and variability on how combustion completes due to the burn rate and time to bullet base muzzle exit affect the variability of muzzle velocity.
I believe it is common practice to "ladder" loads to discover consistency and accuracy of certain combinations of load characteristics during load development. Powder type and charge mass are just some of the variables we ladder, but they are the ones that are the topic of this thread. We've all seen some loads that have a very tight extreme spread and small standard deviation, and other loads that are wildly inconsistent. Consistency in velocity is not solely, maybe not even primarily a function of powder, but it is a factor. Because other factors related to the gun, chamber, barrel, the brass, sizing, crimp, the primer, bullets, and so on can be identified by isolating those variables, I want to discuss and understand how different powder qualities affect consistency.
One factor seems to be "ignitability" -- that is how easily the flakes or balls of powder are ignited. I understand that single base powders are made mostly of nitrocellulose, while double-base powders substitute nitroglycerin for up to 42% of the mass, with most of the remainder being made up of nitrocellulose. With only those ingredients, we would have only "fast burning" powders. Deterrents are added to cause the powders to burn and gasify more slowly, creating pressure more gradually. This allows a greater amount of energy to be delivered within a maximum peak pressure because the energy is delivered over a longer time. But deterrents could make the powder harder to ignite. We may need to use large size primers, "magnum" primers, or rifle primers to get the best combustion of these powders. I realize the distinctions of those different primers is more than just greater ignition -- primers are discussed in other threads. Some obvious examples of hard to ignite handgun powders are H110, IMR4227, and Lil'Gun. But ignitability does not seem to be a binary feature with options for "easy" and "hard." There seems to be a wide range if ignitability among different propellants and the volume and compression of the powder in the case seems to affect the ignitability as well as the pressure during combustion.
Ignitability does not seem to be as much of a problem in rifles with their long barrels. The long barrel gives the powder charge plenty of time to fully ignite when it is under pressure before the bullet base exits the muzzle and the pressure drops rapidly. Because of this, we can use much slower rifle powders. We know if we loaded a handgun cartridge with rifle powder and a bullet and ignited it in the chamber of a short barrel that a large portion of the charge would not complete combustion before the bullet base exited. The pressure would rapidly drop and some of the powder might not even burn at all. It seems that large volumes of slow powders in short barrels create the circumstances where lower "ignitability" creates inconsistent velocities.
I suppose a certain volume of powder -- the volume that a given case restricts us to -- contains so much energy from the NC and NG. The more of that volume that is taken up by deterrent, the less total energy it has. As we go to more and more heavily deterred, slower powders, we need larger and larger cases to hold the greater volume of deterrent without reducing the volume we're filling with NC and NG. So given a fixed case capacity for a particular cartridge, we can only go so slow in burn rate before we start losing energy that could be released within the pressure limit. As we approach this limit, a greater and greater portion of the charge is still combusting when the bullet base exits the muzzle. My conjecture, I don't know if its true, but I think that the smaller the portion of the total charge that finishes combustion before base exit, the greater the opportunity for random variability in the resulting pressure/time curve exists. The greatest consistency would result from a condition where 100% of combustion always completes before base exit. This would cost the opportunity of higher pressures at the time of base exit, and therefore lower overall velocity, but it would best assure consistency in the velocity.
Another factor in the consistency of velocity for handgun cartridges is "position sensitivity." On the other end of the spectrum from large, slow charges we have tiny charges of fast powders. These kinds of loads appear to have what it takes to deliver excellent consistency in velocity unless the case has excess volume. With small charges in large cases like .357 Magnum or .45 Long Colt, we could see differences in the resulting velocity when there is inconsistency with where the powder sits in the case -- against the primer, piled up behind the bullet, or spread out along the length of the case. Again, with short the barrels, it seems more likely we will see differences in the result based on differences in how combustion began. Long rifle barrels seem to even the results out.
Hodgdon advertises its "Titegroup" powder as having characteristics that make it consistent even with small charges in large cases. They claim it has less position sensitivity. They also claim it is more consistent with a variety of differences in primers used. I'm not sure what it is about Titegroup that makes this possible, but it seems to have something to do with "ignitability." Whatever it is, is it restricted to fast-burning powders? Is it restricted to low-volume/low-mass powder charges? Is it a characteristic derived from the portion of nitroglycerin used? I know it is not a result of high NG % alone. Lil'Gun is infamous for having a high NG content, but it is also heavily-deterred, ignites poorly with standard primers, and shows a lot of inconsistency especially in shorter handgun barrels. That brings up the question: because Titegroup is a double-base powder, it seems evident that some double-based powders can produce consistent velocities in handguns. Can single-base powders produce consistent velocities in handguns? The only one I have is IMR4227 and it doesn't seem consistent at all, but it is also exceptionally slow for a handgun powder.
Is there a limit to powder burn rate and barrel length where consistency in muzzle velocity drops away? Can we begin to quantify this limit?
To recap, I think variability in how combustion begins due to the ignitability of the propellent, variability in how combustion proceeds due to the position of the powder in the case, and variability on how combustion completes due to the burn rate and time to bullet base muzzle exit affect the variability of muzzle velocity.
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