I actually prefer nylon + plastic type sheaths. .
Exact same here. The best sheaths I have ever seen were the old Eagle nylon sheaths that came custom-made for the "200 pre-production" Al Mars: They were specifically made for the individual knife, and used rolled-up rubber or plastic with a "hairy" inside, which really prevented scratching.
But even generic nylon sheaths are better if the rolled plastic is a thick and soft type, with adjustable snaps like on the generic-design sheath for the RTAK-II. The adjustable snaps are the key, as they secure the knife tightly at the guard. Unfortunately, these snaps are typically sewn too far from the sheath "mouth" to ensure this: They often have to be re-sewn lower to really offer a rigid hold against the mouth's opening: I just super glue the snaps lower...
As long as the rolled plastic inserts matches the blade thickness tightly (which is where the broadly moulded Spec Ops brand inserts fall short, forcing you to glue inserts inside, a dicey prospect), having a lot of air around your blade is inherently superior from a rust prevention and, especially, a scratch prevention point of view. That is why I never understood the popularity of kydex: The tight form-fit of an unyielding material is the exact opposite of what you want... I have just one knife with Kydex, the Busse Batttlesaw, and the only reason the Kydex doesn't scratch it all to hell is because of its peculiar corrugated surface (and the rough surface finish from being stripped)...
Nylon sheaths also often have pockets that hugely increase the "carrying ability" of the knife.
Second to nylon is black dyed leather, which I have always found to be far superior in avoiding water deformation compared to brown leather (especially when comparing those two options from Randall: Their black sheaths are almost like plastic in comparison): Brown leather sheaths are essentially sponges that will lose their shape completely at the first serious contact with water, like a mild rain... This is so serious brown sheaths should only be considered for inside the waistband carry to try to keep them out of the rain: That really helps. I have one brown leather sheath that works well this way for my carbon Voorhis, but I kick myself for not knowing enough at the time to order it in black, as it was a Patriot leather custom and I had the choice... It kind or goes well with the carbon steel theme, but at least with 5160 I could Cerakoat the steel against rust.
I know some will chime in with Mink oil, silicon or other similar protection, but anything seriously coating the whole inside of the sheath will leave a huge amount of residue on the blade. Black dye in itself reduces water absorption from
within the leather, and most important is the way black leather will not so readily sag when wet like brown leather does. Some black leather is so heavily treated it is literally like soft plastic.
Unfortunately leather is not quite as scratch averse as soft rolled plastic, but both are at their best when the blade has a saber grind surface to "ride" on: The main reason for that is that a saber grind is a "break" in the grinding surface, so you can have the grind striations parallel to the blade friction, something not typically available with a Full Flat Grind blade: In the long run this can make a large difference in the wear appearance from a sheath exposed to dirt and debris.
Gaston
