This method only works when using certain powders that have a linear relationship of muzzle energy vs. powder charge so that powder efficiency and work done by friction can be determined as the slope and vertical intercept of a regression line. If another powder were used, there is some likelihood that the relationship between the powder charge and muzzle energy would not be linear. This method provides an accurate way to measure the average muzzle energy per grain of powder when analyzing the slope of a line for each bullet. Once one has established that the powder being used has a linear relationship, one can use this method to find the work done by friction for the bullet.
It should be noted that many powders can seem to give a linear energy vs. powder charge over a small range, but are unsuitable for estimating barrel friction unless they allow confident extrapolation back to the vertical intercept. Figure 4 shows an energy vs. powder charge graph generated with QuickLOAD V3.6 for H4895 powder and the 60 grain VMAX. (QuickLOAD is an internal ballistics modeling program that predicts muzzle velocities from powder and bullet properties.) At first glance, the data seems linear, and can be fit with a linear function giving a very high R2 of over 0.99. However, more careful analysis reveals a distinct upward curvature of the data, and a quadratic function provides a much better fit. Since the derivative of the energy vs. powder charge curve is not exactly constant, it is not possible to reliably extrapolate back to the vertical intercept to estimate barrel friction by using this powder. In a limited number of cases, QuickLOAD seems to reliably indicate which powders have a sufficiently linear response over a large range of charge weights to confidently extrapolate back to the vertical intercept to determine barrel friction. Some powders (like H4895) gain efficiency as the powder charge is increased. Other powders (usually those with slower burn rates) show diminishing returns as additional powder is added because the powder has not completely burned and the pressure is still high as the bullet leaves the barrel.
Figure 4: Muzzle energy vs. powder charge as predicted by QuickLOAD V3.6 for H4895 and the 60 grain Hornady VMAX bullet, along with the best fit linear and quadratic functions.
Further Study: Testing Claims of Reduced Friction
The results of this experiment provide an accurate, quantitative method for measuring the average barrel friction of the 5.56mm NATO cartridge with various bullets. In addition, it provides a method for testing manufacturer’s claims for reducing barrel friction, such as Norma’s moly bullets (Norma 2011) and SMOOTH-KOTE barrel liner (SMOOTH-KOTE 2006) by comparing the resulting frictions found between treated and untreated barrels and bullets. Bore Tech claims that its drive band technology reduces pressure and fouling thus providing higher velocities for the same bullet weight (Bore Tech 2007). Several patents make claims regarding the friction reducing effects of molybdenum disulfide and tungsten disulfide (Martin 2000) as well as for a proprietary oxide coating using the trade name Lubalox (Stock and Eberhart 2007). The Martin patent (2000) claims that the plating process described allows an increase of bullet velocity by 10%, which would require over a 50% reduction in barrel friction. The authors of the present study are unaware of any published data quantifying these friction reducing claims.
Further Study: Friction of Military Bullets
Analysis of a smaller sample size of velocity data from a separate experiment (armor testing) with the same rifle and the M193 and M855 bullets suggests that while the full metal jacketed M193 bullet had friction comparable with the other 5.56mm bullets at 222 ft-lbs (+/- 21 ft-lbs) of energy lost to friction, the M855 bullet had much larger friction at 412 ft-lbs (+/- 42 ft-lbs) of energy lost to barrel friction. The larger uncertainties are due to the smaller sample sizes and the sealant between the bullet and cartridge case that produces larger shot-to-shot variations in bullet velocity with a given powder charge. The larger friction of the M855 is consistent with the trend of barrel friction increasing with bullet mass, probably attributable to an increase in bearing surface of the longer bullets, but other factors in bullet construction may also contribute. In light of the fact that reducing friction by 50% has the potential to add 200 ft-lbs to the muzzle energy for such a friction prone bullet design, it would seem prudent in the future to understand and mitigate the barrel friction of bullets adopted for military use.
Further Study: Increased Friction Caused by Lead Free Primers
Analysis of a smaller sample size of velocity data from a separate experiment (lead free primers) with the same rifle and lead free bullets suggests a large (>90%) increase in friction when using lead free rather than lead styphnate based primers. Dave Summer, a chemist at the United States Air Force Academy, has suggested (private communication) that the residual lead might be increasing lubricity and that switching to lead free primers might have unintended consequences analogous to engine performance difficulties associated with the switch to lead free gasoline. The impact of lead styphnate vs. lead free primers on friction should be carefully quantified in addition to satisfying previously enumerated criteria (Courtney and Courtney 2011) to ensure a smooth transition to the new technology.
This research was funded by BTG Research (www.btgresearch.org). The authors are grateful for the use of the range owned by Louisiana Shooters Unlimited to collect data. We are also appreciative of the valuable range assistance of Elya Courtney. Dave Summer (USAFA/DFRL) and Lt Col Scott Callihan (USAFA/DFMS) read the manuscript and offered many helpful comments.
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