Excerpt from Fouling Shot 143 Jan/Feb 2000.
Recently, there have been a handful of articles in The Cast Bullet addressing the concept of “lube pumping”. I have learned a great deal from these articles, and I’d like to thank the authors for sharing their thinking with us. In short, their conclusions have been that bullet lube is not just rubbed off of the bullet and onto the barrel, but rather is delivered from the lube groove to the bore via a combination of several mechanisms, including the linear acceleration of the bullet down the bore, the centrifugal acceleration arising from the spin of the bullet after engraving, and the compression of the bullet during firing and the concomitant shrinkage of the groove itself. Without a doubt, each of these mechanisms plays a role, although the magnitude of each of these contributions probably changes as a function of pressure, velocity, alloy hardness, bullet diameter, etc. These concepts also correlate nicely with the research done by the NRA decades ago in which it was the bullet lube’s flow characteristics were found to be important, not necessarily it’s lubricating ability.
I’d like to throw another possible mechanism for delivery of lube from the bullet to the bore out on the table for people to consider. In high-speed photography of a gun being fired, the first thing one sees is a puff of smoke (or even small flames) escaping from the muzzle, significantly preceding the emergence of the bullet from the barrel. This is due to gas blow-by, both from leakage around the bullet in the throat (i.e. pre-engraving) and leakage around the bullet while traversing the barrel (i.e. post-engraving).
Engraving is a brutal process. Bullet metal is distorted and abraded away, and the bullet’s surface undergoes a constant metamorphosis as it travels down the bore. In addition, there are microscopic imperfections in the grooves and lands of the bore. The combination of these microscopic defects creates channels that allow the high pressure gases to escape. These gas molecules are very small (thousands will fit into a defect only .0001” wide), and very hot, and the pressure gradient from the base of the bullet to the first lube groove is extremely high. As soon as these gases reach a lube groove, the lubricant within that groove is subjected to extremely high pressure and temperature. Since the net flow of gas is from the base of the bullet forward, the melted lube is forced forward, precisely where it is needed (the other lube pumping mechanisms leave the lube in the wake of the bullet).
Hard alloys seem to be en vogue these days, especially with the commercial bullet casters. I don’t think I need to preach to this particular congregation that “harder isn’t necessarily better”, nor will you believe me if I tell you that the singular secret to avoiding leading is to use the hardest alloy possible. Sure, hard alloys have their place, but there’s not really much need for ALL cast bullets to have a Brinell hardness of25 or 30. In fact, the commercial casters may well be causing leading by casting all their wares so hard. How? Here are my thoughts. The black powder crowd spends a lot of timemuttering about a bullet “bumping up” in their discussions of internal ballistics (but then again, they spend a lot of time muttering about all sorts of things...). A long time ago, various intrepid experimentalists fired lead bullet loads from barrel-less revolvers into snow banks, oiled sawdust and such. Recovered bullets all showed significant evidence of base expansion. These experiments, while not conclusive, suggest that bullet obturation does indeed take place, given that the alloy is appropriate for the pressures generated. In the intervening years, bullet obturation has become an accepted concept and has been cited as being of central importance to cast bullet success. Some have even gone as far as to measure the bases of recovered cast bullets fired from full-house loads into soft silt, claiming proof of obturation. Two questions always haunt me when I read these accounts: 1) How accurate are these measurements (i.e. how badly deformed was the bullet?)?, and 2) How much of that expansion was due to obturation upon firing and how much was due to deformation upon impact?Glen E. Fryxell
Recently, there have been a handful of articles in The Cast Bullet addressing the concept of “lube pumping”. I have learned a great deal from these articles, and I’d like to thank the authors for sharing their thinking with us. In short, their conclusions have been that bullet lube is not just rubbed off of the bullet and onto the barrel, but rather is delivered from the lube groove to the bore via a combination of several mechanisms, including the linear acceleration of the bullet down the bore, the centrifugal acceleration arising from the spin of the bullet after engraving, and the compression of the bullet during firing and the concomitant shrinkage of the groove itself. Without a doubt, each of these mechanisms plays a role, although the magnitude of each of these contributions probably changes as a function of pressure, velocity, alloy hardness, bullet diameter, etc. These concepts also correlate nicely with the research done by the NRA decades ago in which it was the bullet lube’s flow characteristics were found to be important, not necessarily it’s lubricating ability.
I’d like to throw another possible mechanism for delivery of lube from the bullet to the bore out on the table for people to consider. In high-speed photography of a gun being fired, the first thing one sees is a puff of smoke (or even small flames) escaping from the muzzle, significantly preceding the emergence of the bullet from the barrel. This is due to gas blow-by, both from leakage around the bullet in the throat (i.e. pre-engraving) and leakage around the bullet while traversing the barrel (i.e. post-engraving).
Engraving is a brutal process. Bullet metal is distorted and abraded away, and the bullet’s surface undergoes a constant metamorphosis as it travels down the bore. In addition, there are microscopic imperfections in the grooves and lands of the bore. The combination of these microscopic defects creates channels that allow the high pressure gases to escape. These gas molecules are very small (thousands will fit into a defect only .0001” wide), and very hot, and the pressure gradient from the base of the bullet to the first lube groove is extremely high. As soon as these gases reach a lube groove, the lubricant within that groove is subjected to extremely high pressure and temperature. Since the net flow of gas is from the base of the bullet forward, the melted lube is forced forward, precisely where it is needed (the other lube pumping mechanisms leave the lube in the wake of the bullet).
Properly balanced (i.e. sufficient lube with the proper flow characteristics), this mechanism would have the bullet “swimming in a pool of it own lubricant” as it goes down the bore. Thus, the lubricant forms not only a lubricating layer, but also a fluid gasket between the bullet and bore. The pressure drives the fluid to the defects, where it is needed the most (somewhat akin to a selfsealing tire). In the extreme case (i.e. a severely undersized bullet) there is nothing to restrict the exodus of lube from the lube groove and it could be blasted completely free of the bullet. This is one possible explanation for why undersized cast bullets lead up barrels so badly. With a better bullet-to-bore fit, lube flow is restricted, the lube supply is conserved and maintained at the bullet/bore interface.
Anyway, this is just an idea. I’m hoping that one of you clever people will come up with an enlightening experiment to determine whether this mechanism has any validity.
Ah, the clouds are shifting, the moon is rising, and I feel the next wave of lead-based lunacy coming on...
Anyway, this is just an idea. I’m hoping that one of you clever people will come up with an enlightening experiment to determine whether this mechanism has any validity.
Ah, the clouds are shifting, the moon is rising, and I feel the next wave of lead-based lunacy coming on...
I did a simple little experiment that I feel lends strong support to the concept of bullet obturation, without the complications due to bullet impact. Here’s what I did; using the RCBS 45-255 Keith SWC mould, I cast a batch of bullets with wheelweight alloy (plus a small amount of tin), and a second batch from linotype. The first batch weighed an average of 266grains and the linotype bullets weighed in at 255 grains. All bullets were sized .452” and lubed withmy homemade moly lube (equal parts beeswax and Sta-Lube Extreme PressureMoly-Graph Multi-Purpose Grease). Allbullets were loaded over 9.0 grains of Universal Clays into W-W cases, andprimed with Federal 150 primers. Allchronographing was done within a 1 hour period, under virtually constantweather conditions. The results aresummarized in Table 1.
| Velocity data from .45 Colt trials, RCBS 45-255 Keith SWC | ||||
| Bullet/gun | 3" S&W | 4 5/8" Ruger | 6' S&W | 7 1/2" Ruger |
| Lino (255gr) | 838fps | 887fps | 872fps | 940fps |
| WW (266gr) | 879fps | 942fps | 947fps | 999fps |
Note that the lighter, harder bullet was travelling 41, 55, 75 and 59 fps slower than the heavier, softer bullet in what was otherwise identical ammunition. The same amount of chemical energy was released each time the hammer fell; it’s just a question of how efficiently that energy was converted to work (measured here as velocity). All else being equal, the lighter bullet should end up going faster, and the fact that it was found to be slower indicates that some of the energy was lost (due to gas leakage) in the linotype loads. Presumably, this is due to the fact that these are relatively low pressure loads and the linotype bullets were too hard to “bump up” and seal the bore effectively, whereas the softer wheelweight bullets were able to do so. Clearly, the handgun hunter is better served with the more moderate alloy (more weight and more velocity equals more penetration).
Most commercial bullet casters make a perfectly reasonable product; they are just unnecessarily hard in my opinion. I have shot commercial cast bullets that would severely lead up my revolvers if fired at anything less than full-house magnum pressure levels (stoked to full-throttle, these same bullets left bores shiny and lead free). I just don’t understand why these companies intentionally design and produce a product of such limited utility. (To prevent damage during shipping? Ed.) Oh well, to paraphrase the old song, “That’s all right, I still got my lead-pot....”
One last bit of “howling at the moon”, and what better way to purge the system than to talk about cleaning? As cast bullet shooters, we are constantly working towards that mythical perfect combination of mould, alloy, lube, powder, case and primer that will give us the accuracy and ballistics we desire.... and do it with no leading! Unfortunately, leading is something we cast bullet shooters have to deal with; sometimes only occasionally, sometimes more often.
Many different solutions have been put forward for removing lead deposits from a prized barrel. One that has worked well for me was laid out by Veral Smith in his book on cast bullets. This approach involves cutting off a strip from a copper or brass scrubbing pad and wrapping it around a bore brush. If you’ve never tried this approach for removing leading, you should — it is absolutely amazing! A few strokes and the lead is gone! The drawback to this method is that brass scrubbing pads require a pair of scissors to cut off a piece, and when you do cut it little pieces go flying every which way (which isn’t real popular with either spouses or pets). A much simpler approach, and one that is just as effective, is to replace the scrub pads with bronze wool (not steel wool; bronze wool is available at the larger hardware stores).
I wrap a wet patch around a bronze bore brush and swabout the barrel to first loosen up the powder fouling. Next, I wrap a pinch ofbronze wool around this dirty patch and run it back through the barrel a fewtimes. The bronze wool cuts the leadfouling and the wet patch snags the pieces. Peel off the patch and bronze wool and discard. Wrap a fresh, dry patch around the bore brush and swab out the remaining solvent and fouling. Using this method, it is possible to clean even a heavily leaded barrel in less time than it took to read this description. Less time spent cleaning means more time for casting, loading, shooting and hunting. I like it like that!
Many different solutions have been put forward for removing lead deposits from a prized barrel. One that has worked well for me was laid out by Veral Smith in his book on cast bullets. This approach involves cutting off a strip from a copper or brass scrubbing pad and wrapping it around a bore brush. If you’ve never tried this approach for removing leading, you should — it is absolutely amazing! A few strokes and the lead is gone! The drawback to this method is that brass scrubbing pads require a pair of scissors to cut off a piece, and when you do cut it little pieces go flying every which way (which isn’t real popular with either spouses or pets). A much simpler approach, and one that is just as effective, is to replace the scrub pads with bronze wool (not steel wool; bronze wool is available at the larger hardware stores).
