3. Coatings: Inside the Barrel (chrome, coatings)
What is so surprising about the coatings used on the interior of barrels is that essentially there is only one - CHROME. Everything else I know of is left as bare steel (be it low-alloy or stainless). Given the incredible variety of external coatings you would think at least a handful could/would be used internally, but alas not. Why? Let's dig into that:
A - Line-of-sight
Many coatings rely, at least partially, on a line-of-sight to the substrate to be coated. Applying a uniform coating thickness inside of a 0.300" inner diameter (ID) that is 16"+ in length is DARN HARD TO DO. Heck, chrome plating existed long before someone figured out how to do it inside of a gun barrel. For any coating that would add to the dimensions of the steel you just can't have inconsistency like that inside of the barrel. This shoots down the Ionbond style coatings since you have to sputter deposit them - any hole with a depth greater than one diameter is just not gonna work. Ditto the thermal sprayed tungsten-carbides so common as chrome replacements in other industries - the physics of the spray gun says you can't do a 16" long 30-caliber hole. Chrome only works because they thread a buffered anode the length of the barrel thus ensuring constant anode-to-cathode gap (and thus uniform coating) all the way.
B - High temperature wear
The wear most commonly experienced on the exterior of a gun is holster wear (especially on the front end of the slide). Holster wear is low-temp, low-strain rate and low energy abrasion with some environmental (think humidity and/or sweat) assistance. Conversely, interior barrel wear is high-temp, high-strain rate and high-energy. The difference is between pulling up the bedspread versus having it blasted at you (burning) from a cannon. Thus, many coatings that are pefectly adequate for the exterior (phosphating, bluing, anodizing, paint, electroless nickel) would get obliterated rapidly on the inside of a barrel. While electroless nickel would seem like it satisfies "A" above remember that it gets harder with increased temperature (precipitating those nickel phosphides) so every shot fired would make it harder and more brittle until it cracked/flaked off. Chrome handles it with great aplomb since it doesn't oxidize further (that whole chromium oxide film barrier thingy) and being micro-cracked already gives it places to "flex" kind of like the cuts in a sidewalk.
C - Stuff nobody has thought of ('til now)
Case-hardening has a real place in interior barrel coatings but nobody has done anything with them (at least to my knowledge). Important note - case hardening does NOT increase the base-steel dimensions. The reason why no one has used them is simple - ask a Metallurgist if these processes (carburizing, nitriding, nitro-carburizing and carbo-nitriding) can coat a .300" hole that is 16"+ in length and he'll say, "No way, Jose." Just as in "A" above the coating will not be uniform but rather will be thickest at the end of the holes and virtually non-existent in the middle of the barrel. Ah, but HERE is where you need a Metallurgist who is also a gun nut. I ask you - where is the barrel damage from shooting most pronounced? Is it not the throat and the muzzle? Are these not the two extrema of the barrel ID? Thus, if we could protect JUST THESE AREAS and not necessarily the portion between without affecting ANY dimensions at all, wouldn't we dramatically improve barrel life? Sure would. Or, if we REALLY wanted a uniform coating we could just fill the barrel with powdered charcoal and do an old-fashioned pack carburize - not very high tech sexy but it sure does work. Yeah - I'm gonna have to patent this...
In summary - the interior of a barrel sees shock, high speed wear and rapid temperature spikes that would destroy most coatings that could be applied in such a small space. Rather than engineer a coating around it most gun makers simply figure that a barrel is expendable after a few thousand rounds. Chrome can double the life of a barrel but that's about it. Nothing is forever, ya know.