Confused about the lube groove

[chris in va]

I tumble lube all of my boolits. Obviously it coats the whole thing. Now as the rifling makes contact ahead of the groove, how does a 'normally' lubed boolit work if it's located behind point of contact?

[runfiverun]

it seals the edges of the rifling and the groove allows displaced metal from the boolit somewhere to go.
the boolit ahead of the one you are gonna fire leaves a light film behind which the next one rides along on.
sometimes the lube isn't there [first shot] however lead itself is a type of lubricant.
the antimony on the surface of the boolit will generally leave a trace behind also that is most often seen as a light grey wash it will collect small amounts of lube in the gaps.
if you look at some boolit designs you might be amazed at how little lube they really carry and how much lead is in contact with the bbl.
that also explans why leading generally is not caused by the boolit itself gliding along in the bbl but by something else like gas cutting a mechanical issue or a mechanical/fit between the boolit and the gun.
the last one is how lee t/l type boolits work.
they do not rely on the lube so much, but more on being big/soft enough to mechanically act as a stop leak.

[geargnasher]

Run beat me to it, I said the same thing but not as well, so I scrapped it.

Gear

[btroj]

And how much lube is blown forward of the bullet by the expanding powder gases before the bullet can fully seal the bore?

[runfiverun]

aha that's where the rub is.
there might be none.
quite often i doubt any is blown off in the throat,if it were, none would ever make it into a revolvers bbl.
if you have a fairly tight neck clearance in a rifle none should be blown out there either.

now we have all had some cases come out of a rifle with some lube on the front of them or with dry sooty necks.
what happened there?
poor fit,pressure peak too fast,didn't peak high enough.
lube viscosity is generally the first indicator as the wetter [softer] lubes are the ones that usually coat everything in gloop.
but did it get blown down the bbl?
or just lose it in the throat area?
or is that just a good indicator of too much neck clearance.....

[Eutectic]

aha that's where the rub is.
there might be none.

or is that just a good indicator of too much neck clearance.....

I agree Run. Most of what I shoot boolits in has more neck clearance than I would prefer.

Unless something prevents me from doing it, I seat cast boolits right against the lands. I don't believe you get much lube blown forward this way. Boolit start and engraving start at the same time. I kind of like just a little sooty lube on the fired case. My track record shows this an accurate load far more than otherwise. I think the lube is 'wetting' well in this case and jumping in getting its work done 'in the heat of it'.

Eutectic

[geargnasher]

And how much lube is blown forward of the bullet by the expanding powder gases before the bullet can fully seal the bore?

Chris, I'm going to apologize in advance because this will be a bit of a thread hijack, but Btroj just asked the "million-dollar question" here about lube and it relates to some other work we've been doing. In a way it relates to your question about lube mechanics as well, so bear with me.

Brad, I think the best answer is "some to a lot, rarely none or all, dependent on static boolit fit". In the instance of tight chamber neck clearance and very snug and uniform throat fit, very little lube is blown out ahead of the boolit. If things are sloppy and there is a lot of boolit jump, not only lube but LEAD DUST is blow out. I have several guns that tend to lead the first half to one inch of the grooves if the boolit isn't seated out far enough. I might add that the boolits that lead are bore-riders that don't conform to the throat contour either.

So you can see how lube formula/composition might have a very drastic effect on different gun/load combinations. One guy swears by hard lubes, another by sticky soft ones, but when you really get down to it not many know how well they've truly fit their ammunition to their guns, and tolerances change the way a lube behaves. So does buffer, powder burn rate, and load densities. If you have a very viscous lube and loose fit, you might get purge flyers because of the big blobs of sticky goo that get blown down the bore to be "run over" by the boolit later at high speed. Thinner lubes I would think get blown out more easily, and run over more easily, and you'll find the guys promoting softer lubes usually are very knowledgeable about boolit/case fit.

This is one of the biggest challenges to making "Extreme Lube", more so even than temperature issues. Temperature exacerbates the effect of lube blowout, and different lube formulas handle that blowout differently. I think generally a lube that is relatively temperature/viscosity insensitive, has a very high melt point, is soft and has "glide" to it, and is capable of being spread very thin very quickly even at low temperatures in the event of cold-weather blowout is where it's "at". Still working on it, but I believe Starmetal's lube or a slight variant is going to do that for us.

Gear

[btroj]

I will be starting a new thread about what "fit" means shortly. Need to get my thoughts together.

[44man]

Jacketed is shot with no lube but it is harder and will push junk out of the bore, carbon from powder.
I have never understood why lead needs lube by itself but thinking about keeping junk in the bore soft enough so a softer boolit can move it out instead of running it over kind of makes sense.
The purpose of a good lube in a BP gun is for fouling.
Leading is caused by boolit damage, slump, skid, gas cutting that is mechanical. Lube will not help. It is not a hydraulic pressure stop. If you have gas pressure reach the lube you are done before you start. Lube blown ahead of a boolit means you have no seal. The boolit is the seal, NOT the lube.
Consider the boolit as a hydraulic cylinder wiper that prevents fluid from leaking.
Lube too hard will not coat the bore or soften fouling. Hard lube will throw off a boolit uneven and throw the balance off.
Where does a poor lube fail? Powder is burning behind the boolit, the boolit is laying down some lube. Poor lube will burn with the powder and add more carbon. You will see it as too much smoke at the muzzle. Now you have a dry bore full of more carbon then you want. The next boolit will run over a lot of it.
Comes the BIG question of course. How does lube coat a bore when you have a tight base seal with the boolit?
Hey, I go crazy too because with well over bore size boolits in a BPCR, I get a grease ring on the muzzle. I think it is just lube thrown off at exit. But it really does something in the bore because a poor lube will leave some of the bore dry. Good lube keeps it wet. The best BP lube will absorb moisture.