Since Bob Baker reported his experience with forcing cone erosion on customer guns used with Lil Gun propellant and the results of his experiment evaluating the erosivity of Lil Gun compared to H110, internet-reading magnum users have been shying away from the powder. While Bob postulated that the higher heat he perceived from Lil Gun and it's higher nitroglycerin content may be the cause, I don't think his advice constituted a thorough evaluation of the factors contributing to increased erosion. Therefore, let us consider what makes a propellant more erosive and whether Lil Gun is, in fact, more erosive than other powders with similar performance.
Propellant erosivity is commonly understood to function through three channels: thermal, chemical, and mechanical. The results of semi-emperical correlation studies by Dr. Lawton of the Royal Military College of Science at Cranfield have suggested that in the absence of changes in propellant gas composition, a 10% increase in the maximum temperature results in an increase in erosion of 250%.[1].
The contribution of temperature in erosion must be understood properly if we are to consider any kind of effective mitigation. When a gun is fired, the surface of the bore might reach temperatures well over 2000 degrees Fahrenheit for a few milliseconds, but it quickly cools to a fraction of that temperature after less than a second. The idea of letting a gun "cool off" between shots or even Bob's observation that the gun was too hot to touch have very little to do with the temperatures that cause erosion. The temperatures that are relevant to erosion are measured in thousands of degrees and they only affect a very thin layer of metal on the surface of the bore. This thin layer of metal becomes a "heat-affected zone." Whether you wait a few milliseconds or hours between shots won't have much effect on whether this heat-affected zone is produced. But the maximum flame temperature this area is subjected to can accelerate erosion. That flame temperature is not so much a product of the rate of fire, but of the specific heat of the propellant. That is not to say that the rate of fire cannot increase erosion in the case of a machine gun for example, but that simply slowing the rate of fire does not mitigate thermal erosion in the heat-affected zone.
Reducing the propellant's flame temperature can provide a significant reduction in thermal erosion, but the propellant's specific energy is a direct function of its flame temperature. Consider if Lil Gun is a hot powder, meaning it has a high flame temperature, it also has a high specific energy. Indeed, it delivers high performance in muzzle velocity of projectiles with lower pressure than many of its competitors. In fact, in a number of .357 loads we will need substantially more IMR 4227 and higher pressure to obtain the same projectile muzzle velocities as reduced-loads of Lil Gun. Those increased charges will, in turn, produce more heat. In other words, energy pretty much comes with heat and there's not a lot of heat-free energy. In fact, the powder profiles in Quick Load list the heat of explosion / potential for Lil Gun at 4090 kJ/kg. IMR4227 is 4040 and H110 is 4110. So the notion that Lil Gun is more erosive because it's hotter isn't very instructive. Sure, a small charge of Bullseye is going to result in less heat and less erosion than a case stuffed full of Lil Gun, but it's also going to produce a lot less muzzle energy. We haven't learned anything other than magnum loads erode barrels faster than wimp loads.
But there are two other channels by which erosion happens: chemical and mechanical. If the gas species resulting from the combustion of Lil Gun were more erosive than other propellants like IMR 4227 or H110, then the additional erosion observed with Lil Gun might be occurring through a chemical channel. Chemical channel erosion has been studied by both empirical analysis of experimental firings and through study of the chemical reaction pathways that influence erosion.
Dr. J. Kimura's research published by the American Defense Preparedness Association shows the result of his work to compare the relative contribution of different gas species to gun barrel erosion: CO2 > CO > H2O > H2 > 0 > N2
He also described two different processes: first, lower melting-point compounds, easily eroded by thermal or mechanical means are generated in surface reactions between the gas and bore materials. Second, radial diffusion of gas deeper into the bore material results in interstitial atoms in the lattice of the steel, causing increased brittleness and susceptibility to erosion. This latter process creates a Chemically-Affected Zone which can be as much as tens of microns deep. Erosion of the CAZ can accelerate through oxidation, carburization, hydrogen erosion, embrittlement and cracking.
I don't have the results of mass spectrometry analysis of the gas species produced by Lil Gun vs. H110, but Bob's theory was that the additional nitroglycerin content of Lil Gun was responsible for the increased erosion observed with its use. He postulated it was occuring as thermal erosion, but if this was instead caused by chemical rather than thermal erosion, then we would expect to see more carbon and especially more hydrogen gas constituents in the propellant. But nitroglycerin has only nitrogen and oxygen. Nitrogen gas was even determined by Kimura to have a protective effect. Thus if Lil Gun's content is 10% nitroglycerin, it should have 10% less hydrogen and carbon than a totally gun-cotton based propellant like IMR 4227. In fact, because nitroglycerin has a higher specific energy than nitrocellulose, Lil Gun will have even less hydrogen and carbon for charges of equal energy.
Mechanical erosion is primarily caused by the shear force introduced by sliding friction of the projectile that can remove material, particularly from a cracked, degraded or thermally-softened surface. Mechanical erosion also includes the momentum of of the propellant gas flow with any solid particles entrained within it, resulting in abrasion, sweeping and washing actions. Blow-by of high-pressure propellant gas past the gas-check or driving-band during firing can create jetting, and erosive flow. However, because the effects of mechanical erosion are primarily due to properties of the interface between the projectile and bore, we would not expect to see a dramatic difference with the only variable being the propellant unless the cause of the difference was either thermal or chemical.
Let me state that I admire Bob Baker for taking the time to investigate Lil Gun by conducting his own experiments with a test barrel. I'd like to see more of that kind of inquisitiveness and less simple repetition of something read on the internet. When I heard about Lil Gun, I ordered 4 pounds of it. I'm afraid I can't promise to satisfy anyone's curiosity but my own, but inquiring minds want to know.
[1]. Lawton, B. (1984) Thermal and Chemical Effects of Gun Barrel Wear, in 8th International Symposium on Ballistics, Orlando, United States
[2] Kimura, J.-I. International ballistics symposium and exhibition; 16th, BALLISTICS'96; 1996; San Francisco; CA in BALLISTICS -INTERNATIONAL SYMPOSIUM-; 1; 307-316
Propellant erosivity is commonly understood to function through three channels: thermal, chemical, and mechanical. The results of semi-emperical correlation studies by Dr. Lawton of the Royal Military College of Science at Cranfield have suggested that in the absence of changes in propellant gas composition, a 10% increase in the maximum temperature results in an increase in erosion of 250%.[1].
The contribution of temperature in erosion must be understood properly if we are to consider any kind of effective mitigation. When a gun is fired, the surface of the bore might reach temperatures well over 2000 degrees Fahrenheit for a few milliseconds, but it quickly cools to a fraction of that temperature after less than a second. The idea of letting a gun "cool off" between shots or even Bob's observation that the gun was too hot to touch have very little to do with the temperatures that cause erosion. The temperatures that are relevant to erosion are measured in thousands of degrees and they only affect a very thin layer of metal on the surface of the bore. This thin layer of metal becomes a "heat-affected zone." Whether you wait a few milliseconds or hours between shots won't have much effect on whether this heat-affected zone is produced. But the maximum flame temperature this area is subjected to can accelerate erosion. That flame temperature is not so much a product of the rate of fire, but of the specific heat of the propellant. That is not to say that the rate of fire cannot increase erosion in the case of a machine gun for example, but that simply slowing the rate of fire does not mitigate thermal erosion in the heat-affected zone.
Reducing the propellant's flame temperature can provide a significant reduction in thermal erosion, but the propellant's specific energy is a direct function of its flame temperature. Consider if Lil Gun is a hot powder, meaning it has a high flame temperature, it also has a high specific energy. Indeed, it delivers high performance in muzzle velocity of projectiles with lower pressure than many of its competitors. In fact, in a number of .357 loads we will need substantially more IMR 4227 and higher pressure to obtain the same projectile muzzle velocities as reduced-loads of Lil Gun. Those increased charges will, in turn, produce more heat. In other words, energy pretty much comes with heat and there's not a lot of heat-free energy. In fact, the powder profiles in Quick Load list the heat of explosion / potential for Lil Gun at 4090 kJ/kg. IMR4227 is 4040 and H110 is 4110. So the notion that Lil Gun is more erosive because it's hotter isn't very instructive. Sure, a small charge of Bullseye is going to result in less heat and less erosion than a case stuffed full of Lil Gun, but it's also going to produce a lot less muzzle energy. We haven't learned anything other than magnum loads erode barrels faster than wimp loads.
But there are two other channels by which erosion happens: chemical and mechanical. If the gas species resulting from the combustion of Lil Gun were more erosive than other propellants like IMR 4227 or H110, then the additional erosion observed with Lil Gun might be occurring through a chemical channel. Chemical channel erosion has been studied by both empirical analysis of experimental firings and through study of the chemical reaction pathways that influence erosion.
Dr. J. Kimura's research published by the American Defense Preparedness Association shows the result of his work to compare the relative contribution of different gas species to gun barrel erosion: CO2 > CO > H2O > H2 > 0 > N2
He also described two different processes: first, lower melting-point compounds, easily eroded by thermal or mechanical means are generated in surface reactions between the gas and bore materials. Second, radial diffusion of gas deeper into the bore material results in interstitial atoms in the lattice of the steel, causing increased brittleness and susceptibility to erosion. This latter process creates a Chemically-Affected Zone which can be as much as tens of microns deep. Erosion of the CAZ can accelerate through oxidation, carburization, hydrogen erosion, embrittlement and cracking.
I don't have the results of mass spectrometry analysis of the gas species produced by Lil Gun vs. H110, but Bob's theory was that the additional nitroglycerin content of Lil Gun was responsible for the increased erosion observed with its use. He postulated it was occuring as thermal erosion, but if this was instead caused by chemical rather than thermal erosion, then we would expect to see more carbon and especially more hydrogen gas constituents in the propellant. But nitroglycerin has only nitrogen and oxygen. Nitrogen gas was even determined by Kimura to have a protective effect. Thus if Lil Gun's content is 10% nitroglycerin, it should have 10% less hydrogen and carbon than a totally gun-cotton based propellant like IMR 4227. In fact, because nitroglycerin has a higher specific energy than nitrocellulose, Lil Gun will have even less hydrogen and carbon for charges of equal energy.
Mechanical erosion is primarily caused by the shear force introduced by sliding friction of the projectile that can remove material, particularly from a cracked, degraded or thermally-softened surface. Mechanical erosion also includes the momentum of of the propellant gas flow with any solid particles entrained within it, resulting in abrasion, sweeping and washing actions. Blow-by of high-pressure propellant gas past the gas-check or driving-band during firing can create jetting, and erosive flow. However, because the effects of mechanical erosion are primarily due to properties of the interface between the projectile and bore, we would not expect to see a dramatic difference with the only variable being the propellant unless the cause of the difference was either thermal or chemical.
Let me state that I admire Bob Baker for taking the time to investigate Lil Gun by conducting his own experiments with a test barrel. I'd like to see more of that kind of inquisitiveness and less simple repetition of something read on the internet. When I heard about Lil Gun, I ordered 4 pounds of it. I'm afraid I can't promise to satisfy anyone's curiosity but my own, but inquiring minds want to know.
[1]. Lawton, B. (1984) Thermal and Chemical Effects of Gun Barrel Wear, in 8th International Symposium on Ballistics, Orlando, United States
[2] Kimura, J.-I. International ballistics symposium and exhibition; 16th, BALLISTICS'96; 1996; San Francisco; CA in BALLISTICS -INTERNATIONAL SYMPOSIUM-; 1; 307-316
