For the lock , you can remove the bluing or in the case of case hardening , polish it out , then brown .
Plumb brown can be used but know that the quality of finish can be just as bad as are slow rust if the surface isn’t prepared properly . IE you can end up with that rough surface like is achieved on a slow rust . A good rust brown should be even and smooth . a lot of times folks think that smoothness is derived from a high polish of a part . Fact is that with a rust brown , there isn’t much need to go past 300 grit .
The smooth polished surface comes from carding off the heavy oxide , leaving only the very fine stain size oxide .
that’s where Plumb brown shines as there isn’t any real need for repeated carding . However I have found it to be not a durable as a slow rust in that over time and use , you can end up with a somewhat blotchy surface .
As to your barrel .
New barrels normally don’t come finished to the point they are ready for rusting or bluing .
that’s where draw filing comes in . draw filing removes not only the tooling marks but also light pitting and gives you an even surface prep to start from .
Again when finishing a barrel you need to consider what treatment your going to use .
When choosing to brown , you can actually polish a barrel to the point it inhibits the formation of rust .
Be it a rust black , blue or brown , its all derived from the formation of iron oxide .
As such again no need to really go past 300 grit .
But if one wishes to use say a niter blue or a blue “cold blue “or brown stain then you want a better finish level . Especially concerning niter or fire blues.
What I would do with your barrel is draw file it back . Then hit it would emery paper on a good sanding block so as to keep the corners of the flats good and sharp . Then do you brown . Be it plum brown or a slow rust
