NP3 and derivatives are basically PTFE infused Nickel-Phosphorous, the nickel makes it hard, the phosphorous makes it corrosion resistant, and the PTFE gives it low friction... but also makes it wear faster and have lower temperature stability.
NP3 is good for the outside of the barrel (not great), but not good for the inside, since it will wear faster when exposed to the combustion gasses and bullet friction. NP3 is used on a lot of firearms parts, primarily where you want low friction and corrosion resistance, but where heat or hard wear is not a problem. The PTFE also makes it less likely to chip. Think magazines, safeties, hammers, disconnectors, firing pin blocks.
For a pistol barrel with standard civilian round counts it will probably be fine in terms of wear, but it is a no-go on rifle barrels for sure.
Chromium Nitride is like hard chrome, but is twice as hard and a has lower friction, and VERY high temperature stability, making it optimal for barrels. It is also applied in a manner that does not reduce accuracy, unlike chrome plating.
DLC or TaC is very hard and very low friction, but I believe it is more likely to chip, and since it is carbon, a microscopically thin layer can actually oxidize over time or at high temperatures. My AR bolt carrier and firing pin are DLC coated.
