The jury is still out on all the varieties of super-duper EP additives in bullet lube. Most engine assembly lubes contain some form of organo-metallic zinc and/or molebdenum compounds (either with sulphur or other complexes) which serve as 'boundary lubricants', meaning they will prevent metal galling well past the pressure point when lubricating oils are squeezed out or burned away and can no longer provide the normal dynamic film lubrication that oils do. Sounds good in a rifle barrel, doesn't it? If a soft lead bullet zinging down a rough steel tube with blistering-hot propellant gas behind it isn't a boundary condition I don't know what is. The problem is that bullet lubrication is unique in the world of tribology and there isn't an industrial analogy for which machinery lubricants have been developed. Bullet lube has some very unique requirements, most of which we have only a very limited, empirical understanding.
In the world of moving metal parts, the closest thing we have to compare to a bullet-barrel interface is the babbit bearing on a polished steel journal. For low-speed, heavy load, high heat applications such as kiln bearings, an open well is used to apply stiff sodium brick grease constantly and directly to the journal itself. For high-speed babbit, thin oil is injected into the bearing directly under pressure through lubrication passages, and typically the journal has a lateral load and some engineered clearance to allow the oil to enter from the off-side, where there is clearance, and provide a constant supply to be wicked between the surfaces at the pressure point. Engine crankshaft and camshaft bearings operate this way. With bullets, we can afford neither clearance nor reliable pumping pressure, and the velocity is several hundred times the surface speed that most greases or even oils are rated for. The only reason bullet lube works at all is the distance and duration is very short.
Research devoted to bullet lubrication has pretty much been limited to hobbyists like us with little background in fluid mechanics and even less access to laboratory equipment capable of measuring meaningful values, so the research is both painfully slow and often times inconclusive. Even the experiments the NRA did half a century ago aren't extremely relevant today when many of use are approaching or exceeding 3,000 fps with our rifles. Not many lubes deliver the goods much past 2200 fps, so we're on our own here. One thing that Col. Harrison of the NRA determined that I do tend to agree with, and is relevant to this conversation, is that any sort of "solid" in a bullet lube isn't necessarily desirable. I personally don't like graphite or molybdenum disulphide in my lubes, such things are tempermental, can be forced out of the mix under high pressure, leave inconsistent deposits in the bore, and really don't seem to help the boundary lubrication aspects of the system very much. Soft lead bullets bearing on hardened steel have different requirements than two hardened surfaces bearing on one-another, which is what these extreme-pressure additives in greases, assembly lubes, and lubricating oils are engineered to do. One of the best lubes ever invented is the OLD NRA formula of equal parts beeswax, paraffin wax, and Vaseline, which contains no semblance of "EP" additives aside from the mono- and di-esters contained in the beeswax. Zinc dithiodiphosphate (ZDDP) is a common oil additive and camshaft break-in lube which remains in suspension under all but the most demanding conditions. It is what makes STP oil treatment what it is, thought the EPA has virtually mandated it out of existence for engine use because it is damaging to exhaust catalyst beds and is on the emissions "hit list". Certain polymer compounds are being used to replace it, Fuchs oil company being one of the pioneers in this respect. I can't really tell if ZDDP is good or bad for bullet lube, in fact I don't think it makes as much difference as the type of base oil in which it's dissolved does for the lube. Something about those soft lead bullets seems unaffected by these advanced boundary lubricants. Wax and oil seem do do the job just fine, especially when buffered with a lot of metallic soap thickener to control melt and flow. Basically, bullet lubrication needs to function on the "dynamic film" side of things, that means floating the metals on a wet film to keep them from contacting. We do this by keeping gas sealed behind the bullet so it doesn't wash away the film, and making a lube that flows just enough but not too much and thins predictably as the bullet accelerates. The bullet needs a very high-strength, stiff film to protect it during engraving process (and a slight bit of gas leakage right at launch which blows a littel lube ahead of the bullet helps here), yet the lube needs to be thixotropic and thin under pressure/shear so that it can 'keep up' with the needs of the bullet as sliding speeds approach several hundred feet per second farther down the bore. Another trick is to engineer the lube so that, as pressure drops off toward the muzzle, it doesn't get "dumped" in the bore as the bullet "relaxes" and fit tolerances increase.
Ever hear the phrase "running out of lube" used to express the reason for lead streaks appearing only toward the muzzle end of the bore? I don't believe in such a term. Lubrication failure it might be, but lack of lube I doubt. What tends to happen, and anyone can verify this the same way I did by adjusting powder burn rate or alloy composition, is that gas leaks form around the bullet as pressure drops off, allowing gas cutting and lead dust deposits. A lube that lacks sufficient viscosity to resist this relax-point blowout will experience dynamic film failure and contact abrasion can also occur from that point forward in the bore. Metal salts like lithium or sodium stearate help control viscosity, flow, and blowout. So do slow powders and resilient, springy alloys that resist relaxing. Another thing that helps prevent muzzle-end leading is using an alloy that resists particle abrasion. Antimony has a bad habit of sloughing off in the bore when used in excess, particularly on the leading edge of the lands at steep approach angles and high speeds, and can abrade leaving gaps for the lube to blow out causing leading from increased abrasion and gas cutting. There isn't a high-tech additive I know of that can prevent this sloughing or keep it from sticking to the bore. If I find one you guys will be the next to know.
. Have you worked with engine assembly lube, like that offered by Clevite and Federal Mogul? The stuffs pretty sticky, and I've been wondering if Lucas Red Grease is related to it somehow.
