I've been asking myself this question, and I'm not sure if I'm any further ahead as a result. It at least passes the time while I'm driving.
I've read that bullet lubes (and lube grooves) get "shoved back" or "Squeezed out" of the bullet grooves due to the extremely fast acceleration upon powder ignition. I have read that lube grooves on bullets may have some benefit to not be square in cross section, but angled back ("V shape" to some degree) or round, to facilitate the "the lube squeeze" out upon "detonation acceleration."
Here's why I started to think more to it. Upon powder ignition, gases expand and the bullet accelerates forward. Does any of the lube get "pushed forward" by the gas (as in gas blow by / gas cutting) that would lube forward?
I look at various bullet designs and wonder how they work. First, is a "true Keith" bullet. Square lube grooves, and 3 equal groove-deep bands. The lube goove is behind two of the leading bands. In essence, you are only lubing the last 1/3 of the bullet. What is lubing the first two bands? Why no leading? Is this due to "gas push forward" or "gas cutting" lubing? I don't know.
Then I looked at some cast bullet designs (I'm playing with some online designs at Mountain Moulds for a 9.3/.366" ~ 250 - 300 grain gas check bullet for my 9.3x62). I has a short neck (as does the 35 Rem, and 300 Savage which I also have). How do you have a large bore riding surface, and avoid leading?
So I ask the distinguished readers of castboolits.com to shed their views. Just how in the world does lubing work? If you can NOT lube for 2//3 of a Keith bullet, and only lube the bottom third of the bullet, there must be some other factor involved. Maybe there is something to a tumble lubed bullet.
If the only factor in lubing was the "acceleration squeeze out" factor, you'd think a lube groove pattern that has WIDER grooves up near the crimping groove, and then narrower lube grooves as you get toward the base of the bullet would make the most sense.
Enlighten me.
Thanks in advance.