Too much coffee and a day off
Jim - Thank you for posting the link to the USAF sponsored bullet coatings testing paper. Very interesting reading. While as a scientist I appreciate the value of conclusions based on empirical evidence, particularly from well-executed testing with carefully controlled variables, after review of this document, and review of the report of the development of the method used, I have come to the following conclusions:
1 This method may or may not measure “bullet/barrel friction”. Quite possibly not, at least as it relates to precision shooting applications. This method assumes a total barrel friction value from the extrapolation of powder/velocity curves to the y-intercept (y-axis = muzzle energy). NO DATA comparing this “new method” to existing methods is provided. (If you are familiar with alternate analytical method development, you recognize this as a very significant issue with this publication.) There is even contradictory data within this study. Of the three bullet types/brands tested, moly and WS2 coating purportedly produced increased friction in two bullet types, but reduced friction in another. HBN coating decreased friction in two bullet types (not the same two as above), but did not in the other bullet type (two values are well within published “uncertainty” in the method).
2 The relationship between the calculated (not directly measured) “bullet/barrel friction” in a system consisting of a (presumably?) stock Remington 700 barrel (chrome moly?, button rifled? hammer forged? non-lapped?) shooting what are significantly reduced loads of Blue Dot in a .223 cartridge to the shooting of a full power load with a powder more likely used in precision shooting in a high quality 416R SS hand-lapped barrel might be quite tenuous, if not completely irrelevant. This "friction" measurement method didn’t work with other powders and loads. For example, this method did not work with reduced loads of H4895, my powder of choice for .223 and 22BR. Not to mention full power loads, which also did not work with this method.
3 This test concludes that due to the lack of significant reduction in barrel friction, it would not be efficient to coat bullets in military applications for the purpose of extending barrel life. Even if the analytical method used does in some way measure “barrel friction”, the level of friction of a bullet travelling down a rifle bore likely has little to no relationship to “barrel life” in precision shooting applications, and probably little relationship to precision. What would likely be highly significant to both barrel wear and precision is the shape of the friction curve as the bullet engages the bore, i.e. the first ¼” to 1” or so of bullet travel.
Pretty much accepted? - Barrel wear in precision shooting (as opposed to military applications) is mostly due to two issues: 1) metal cracking and erosion in the throat area caused by repeated exposure to the high temps of the initial powder burn, and 2) abrasion of the barrel crown likely caused by repeated cleaning brush contact. Precision shooting barrels don’t really wear out down the bore. The pattern of barrel erosion is maximum to minimum, throat to muzzle.
If the requirement for cleaning is minimized through a reduction in copper deposition in the barrel achieved through the use of moly/WS2/H-BN coating, as has been thoroughly demonstrated by a multitude of precision shooters over many years, then the use of coated bullets is likely to extend barrel life due to crown wear. But precision re-crowning of a good barrel is common, is relatively inexpensive, and can quickly restore crown performance. So this is not really that much of an issue.
Barrel throat erosion is much more significant. The temperature reached by a powder mixture burn and the generation of hot gases will be directly proportional to the pressure. Higher pressure = higher temperature. (Pressure level can even effect the rate of burn.) Higher temperatures will erode a barrel throat faster. Possibly MUCH faster, depending on a multitude of factors. As a bullet moves out of the case neck and engages the lands the volume of the combustion chamber increases. Increased volume = less pressure (all other factors equal). Therefore, if given the same system (powder load, bullet, barrel), if the bullet moves out of the case neck and into the barrel with less pressure (as the burn continues), the maximum temperature reached in the throat area will likely be significantly less. If bullet coating can achieve this, then, shot after shot, the sensitive throat area will be exposed to significantly lower barrel metal-eroding temperatures. If the bullet exhibits more or less “friction” further down the barrel, this will be pretty much irrelevant to barrel wear as pressures and temperatures are already significantly lower.
Again, what is likely most relevant is how quickly and with what pressure level can the bullet be made to move into the throat and down the initial portion of the barrel, increasing combustion chamber volume and thereby lowering throat eroding temperatures. Most shooters report that to achieve similar muzzle velocities with coated vs. uncoated bullets, and increase in powder charge is required, supporting the hypothesis that the use of moly coated bullets results in reduced barrel pressures. If a significant portion of this pressure reduction occurs within the first ~1” of bullet movement, this is likely to significantly reduce barrel wear. Even if powder charge is increased to achieve equal/increased muzzle velocity, I would think (no evidence to support this) that the pressure curve would still show a lower maximum pressure/temperature in the critical ~1” of bullet movement.
This brings me to a story . . .
Many years ago someone at a range had a cut off barrel stub with the end of the chamber exposed. He had coated and uncoated bullets, and challenged people to push the bullets (as I recall, he actually had a small nylon mallet) into the bore. The uncoated bullets tended to go in somewhat, but visibly crooked, non-concentric bullet “cylinder” to bore. Moly coated bullets simply could not be made to enter the barrel in a non-concentric orientation. The moly coated bullets always “slipped” into the correct orientation, and with much less force. (If someone else remembers this, please post your recollections.)
How that observation relates to firing a bullet in a barrel is unknown, but precision shooters, particularly benchrest shooters, go to great lengths to ensure bullet to bore concentricity, and it was a powerful demonstration.
Now the “faith-ish” part.
Based on the above observations, it is possible or likely that the use of coated bullets results in an improved bullet to barrel bore concentricity when fired (if that is the correct term), and will deform less due to lower pressures due to lower pressure throat entry. This may in-turn result in less initial bullet movement as it exits the muzzle and “goes to sleep”, a decrease the effect of crosswind, and improved downrange precision. While the less cleaning thing and extended barrel life, which, at least for me, appear to be real, are significant advantages, the primary reason I coat my bullets is to improve the quality of bullet-bore orientation and reduce bullet deformation. The images shown in coyotechet’s post above are also quite striking, with non-coated bullets showing significant shape disruption vs. the coated bullets. (Apparently similar coated vs. uncoated bullet deformation images have been previously published.) Of these bullet shapes, I would think that the coated bullets, with less deformation, are likely to stabilize faster, fly more precisely and possibly be less affected by crosswinds.
Please note I have “no skin in this game”. I’ve just posted this out of technical interest’s sake.
KC
Jim - Thank you for posting the link to the USAF sponsored bullet coatings testing paper. Very interesting reading. While as a scientist I appreciate the value of conclusions based on empirical evidence, particularly from well-executed testing with carefully controlled variables, after review of this document, and review of the report of the development of the method used, I have come to the following conclusions:
1 This method may or may not measure “bullet/barrel friction”. Quite possibly not, at least as it relates to precision shooting applications. This method assumes a total barrel friction value from the extrapolation of powder/velocity curves to the y-intercept (y-axis = muzzle energy). NO DATA comparing this “new method” to existing methods is provided. (If you are familiar with alternate analytical method development, you recognize this as a very significant issue with this publication.) There is even contradictory data within this study. Of the three bullet types/brands tested, moly and WS2 coating purportedly produced increased friction in two bullet types, but reduced friction in another. HBN coating decreased friction in two bullet types (not the same two as above), but did not in the other bullet type (two values are well within published “uncertainty” in the method).
2 The relationship between the calculated (not directly measured) “bullet/barrel friction” in a system consisting of a (presumably?) stock Remington 700 barrel (chrome moly?, button rifled? hammer forged? non-lapped?) shooting what are significantly reduced loads of Blue Dot in a .223 cartridge to the shooting of a full power load with a powder more likely used in precision shooting in a high quality 416R SS hand-lapped barrel might be quite tenuous, if not completely irrelevant. This "friction" measurement method didn’t work with other powders and loads. For example, this method did not work with reduced loads of H4895, my powder of choice for .223 and 22BR. Not to mention full power loads, which also did not work with this method.
3 This test concludes that due to the lack of significant reduction in barrel friction, it would not be efficient to coat bullets in military applications for the purpose of extending barrel life. Even if the analytical method used does in some way measure “barrel friction”, the level of friction of a bullet travelling down a rifle bore likely has little to no relationship to “barrel life” in precision shooting applications, and probably little relationship to precision. What would likely be highly significant to both barrel wear and precision is the shape of the friction curve as the bullet engages the bore, i.e. the first ¼” to 1” or so of bullet travel.
Pretty much accepted? - Barrel wear in precision shooting (as opposed to military applications) is mostly due to two issues: 1) metal cracking and erosion in the throat area caused by repeated exposure to the high temps of the initial powder burn, and 2) abrasion of the barrel crown likely caused by repeated cleaning brush contact. Precision shooting barrels don’t really wear out down the bore. The pattern of barrel erosion is maximum to minimum, throat to muzzle.
If the requirement for cleaning is minimized through a reduction in copper deposition in the barrel achieved through the use of moly/WS2/H-BN coating, as has been thoroughly demonstrated by a multitude of precision shooters over many years, then the use of coated bullets is likely to extend barrel life due to crown wear. But precision re-crowning of a good barrel is common, is relatively inexpensive, and can quickly restore crown performance. So this is not really that much of an issue.
Barrel throat erosion is much more significant. The temperature reached by a powder mixture burn and the generation of hot gases will be directly proportional to the pressure. Higher pressure = higher temperature. (Pressure level can even effect the rate of burn.) Higher temperatures will erode a barrel throat faster. Possibly MUCH faster, depending on a multitude of factors. As a bullet moves out of the case neck and engages the lands the volume of the combustion chamber increases. Increased volume = less pressure (all other factors equal). Therefore, if given the same system (powder load, bullet, barrel), if the bullet moves out of the case neck and into the barrel with less pressure (as the burn continues), the maximum temperature reached in the throat area will likely be significantly less. If bullet coating can achieve this, then, shot after shot, the sensitive throat area will be exposed to significantly lower barrel metal-eroding temperatures. If the bullet exhibits more or less “friction” further down the barrel, this will be pretty much irrelevant to barrel wear as pressures and temperatures are already significantly lower.
Again, what is likely most relevant is how quickly and with what pressure level can the bullet be made to move into the throat and down the initial portion of the barrel, increasing combustion chamber volume and thereby lowering throat eroding temperatures. Most shooters report that to achieve similar muzzle velocities with coated vs. uncoated bullets, and increase in powder charge is required, supporting the hypothesis that the use of moly coated bullets results in reduced barrel pressures. If a significant portion of this pressure reduction occurs within the first ~1” of bullet movement, this is likely to significantly reduce barrel wear. Even if powder charge is increased to achieve equal/increased muzzle velocity, I would think (no evidence to support this) that the pressure curve would still show a lower maximum pressure/temperature in the critical ~1” of bullet movement.
This brings me to a story . . .
Many years ago someone at a range had a cut off barrel stub with the end of the chamber exposed. He had coated and uncoated bullets, and challenged people to push the bullets (as I recall, he actually had a small nylon mallet) into the bore. The uncoated bullets tended to go in somewhat, but visibly crooked, non-concentric bullet “cylinder” to bore. Moly coated bullets simply could not be made to enter the barrel in a non-concentric orientation. The moly coated bullets always “slipped” into the correct orientation, and with much less force. (If someone else remembers this, please post your recollections.)
How that observation relates to firing a bullet in a barrel is unknown, but precision shooters, particularly benchrest shooters, go to great lengths to ensure bullet to bore concentricity, and it was a powerful demonstration.
Now the “faith-ish” part.
Based on the above observations, it is possible or likely that the use of coated bullets results in an improved bullet to barrel bore concentricity when fired (if that is the correct term), and will deform less due to lower pressures due to lower pressure throat entry. This may in-turn result in less initial bullet movement as it exits the muzzle and “goes to sleep”, a decrease the effect of crosswind, and improved downrange precision. While the less cleaning thing and extended barrel life, which, at least for me, appear to be real, are significant advantages, the primary reason I coat my bullets is to improve the quality of bullet-bore orientation and reduce bullet deformation. The images shown in coyotechet’s post above are also quite striking, with non-coated bullets showing significant shape disruption vs. the coated bullets. (Apparently similar coated vs. uncoated bullet deformation images have been previously published.) Of these bullet shapes, I would think that the coated bullets, with less deformation, are likely to stabilize faster, fly more precisely and possibly be less affected by crosswinds.
Please note I have “no skin in this game”. I’ve just posted this out of technical interest’s sake.
KC
