Nitre bluing isn't as durable as either rust bluing or hot bluing. For the wear parts you're talking about, I'd have them hot blued. I nitre blue, but only as a convenience to myself on small parts like screws. I send out hot bluing instead for most parts, and rust blue barrels for traditional single shots. Nitre blue salts is also a good medium for drawing out and tempering springs if you have good temperature on your pot and a thermocouple to accurately measure temperature.
