Bullet "Lube" - Does it lubricate or Seal or Something Else?

John,

You ask a very good question. My opinion is lubrication. My reasoning is like this. My cast bullets measure 0.3095 and I shoot them in my 0.308 barrel. Really that ought to form a great seal all by itself whether we gently inject the projectile into the barrel or slam it in at several hundred FPS. I think all surfaces of barrel interiors have some measurable roughness and I think the lubricant prevents excessive wear of the bullet surface, (leading) to occur in those micro cavities and scratches. Something like automotive pistons. The rings seal and oil that is held micro cavities of the piston shaft to lubricate and prevent wear.  I’m sure that professional tribologists might have a more sophisticated explanation. I worked with tribologists at MIT and RPI to understand the science of friction and wear pertaining to the action of abrasives on metals in the presence of a variety of organic polymers. 

A few days ago, I was sizing some newly cast bullets that were 0.3105 diameter. I was curious about the length increase by sizing. As I recall, it was a few ten thousands, not important for my point. When my  bullet at 0.3095 is further sized in my 0.308 barrel, I think the deformation process exerts a force on the projectile that tries to fill all void spaces. A rough barrel has many more void spaces and without adequate lubrication will accept lead deposits more easily than a smooth barrel.

The lubricant could act as a seal, but that argument gets into a rheology discussion. My guess is that our type of lubricants don’t undergo shear thinning like a paint dispersion does. I would expect the opposite behavior. The force to deform will increase as the rate of shear increases.

Loren