You're correct,,it's just like rust blueing in that no matter how long you do it, it always seems to be able to throw you a curve even if just once in a while. Plus a printed 'how-to' can get you started, but that good old experience is the only real way to get it down. Everyone has their own little tricks, techniques, ect that gets them their best results and they rarely match the other guys way of doing this 100%. But both can turn out equally nice work. Nice details on that Marlin. I like the use of different finishes to compliment the complete project.

That Hepburn goes back to the owner soon. I have a feeling I'll be getting a barrel, butt plate and some screws to do some work on. Thank you for the compliments on the work. It's one I'll probably never see the finished rifle though. Other hands doing work on it (wood, finish, etc) and not in what I'd consider anything of a correct order!,,but it's their gun..

The Carbonia process may not fully anneal 4140, but when dealing with and refinishing older firearms, most of the time you have little or no idea what the steel is. Nor do you know what the heat treat characteristics are, what temp/time they were drawn back to, or how what you are about to do may alter what has been done by the factory as an established safety criteria for a heat treated part.

A part made from 4140 that was originally drawn back at under 800F, now subjected to 800F during the process is not going to maintain the same strength in all likelihood. Different applications call for different degrees of tempering. Without knowing what the original steel is in some cases, let alone the spec for tempering is just guessing from there on out. Annealing is the complete removal of applied hardness to the metal. Tempering is removing some of the hardness on a known scale to maintain tougness w/o brittle structure. Material data sheets and Machinists books will give the rundown on what a known steel will harden and draw back to when subjected to certain temps and times. That's helpful info but it won't tell you what a gun part is supposed to be in terms of hardness (if you do happen to know what steel it's made of). Mfg's are stingy with steel, heat treat info and methods.

It's not an area you want to guess in my opinion,,but I've been known to be conservative when it comes to stuff like this.
Hell, people get upset if they see purple/blue temper colors from an over zelous soft soldering job on a barrel or receiver and reject the gun that the steel strength has been compromised. What about putting the same part in an oven at 800F for a couple of hours, then letting it cool. I think that may, in some cases, change things.

Just got to be careful that's all when doing stuff like this and think about what you may be doing to that steel before proceding. You don't want to make a potential safety hazard and put into someones hands. Certain firearms,, mostly of a more modern pursuasion, just shouldn't be subjected to it IMHO.

I feel you should stick to parts that would have originally had or were available in these types of finishes. Obtaining a finish like these just for the sake of it on 'critical' parts, safety be damned, is foolish.

The blueing processes will just make them softer than before,, if anything. Not like case coloring where you can potentially create an overly hardened piece. If you think they never get warpage or cracked pieces,,think again. A Browning 86 out of the quench cracked totally in half is quite a sight. There's that unkn modern steel again.