Hard chromium is tough as well as hard. It is plated onto an oversize bore so that the plate thickness doesn't constrict the rifling. I don't know if nitriding is tough or not? It's hard, but it may be too brittle for the pressures and temperatures near the throat, promoting premature surface cracking. I just don't know, but I wouldn't just assume it's a good idea until I checked with a metallurgist about it.
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