Tetra lube is the equal of friction block, which has been discontinued. Tetra is available from many gun shops, in both oil and grease forms and both are excellent.
I don't believe it soaks into the metal but fills microscopic surface imperfections. The only thing I know of that will penetrate or pass through steel is Hydrogen gas, which says that steel is not actually a solid material, but that it has a pore structure which is too fine for most things to penetrate. So maybe it does penetrate the steel a bit.
Don't apply it if you think you might want to lap the barrel. It resists abrasion so well that lapping is nearly impossible, until the tetra has worn away from maybe a thousand rounds or so!
Tetra is compatable even with water, so I wouldn't hesitate using it with black powder guns, wherein the black powder is water soluable, unlike smokeless powders.
