There is a possible issue with nitrided bolt carriers. Well, more than one, but I'm going to discuss one of them here.
Usually, when the carrier and gas keys are machined, they leave the bores oversized so that they can be hard chromed, which is a relatively thick finish as finishes go. The thickness of the hard chrome makes up for the difference in the oversized bores, and brings them down to the proper dimensions.
Nitriding does not add thickness. It penetrates the surface down to a certain depth, and does not sit on top of the surface.
In many nitrided bolt carriers, you end up with oversized bores in the gas key and the carrier. These oversized bores result in gas leakage around the gas tube and the bolt.
Usually, when the carrier and gas keys are machined, they leave the bores oversized so that they can be hard chromed, which is a relatively thick finish as finishes go. The thickness of the hard chrome makes up for the difference in the oversized bores, and brings them down to the proper dimensions.
Nitriding does not add thickness. It penetrates the surface down to a certain depth, and does not sit on top of the surface.
In many nitrided bolt carriers, you end up with oversized bores in the gas key and the carrier. These oversized bores result in gas leakage around the gas tube and the bolt.