Different metals take vinegar bluing differently.

I see what looks like an sks bolt set and a Mosin-Nagant bolt and trigger guard/Mag well?

I’m certainly no expert, but I’ll surmise different component metals as well as different heat/hardness treatments on the various parts may be a cause.

Stay safe.

In my experience, a vinegar bath is a good way to remove bluing. When I was doing Civil War reenactments, we would soak reproduction Enfield musket barrels (blued) in vinegar to make them "bright." But too much soaking would etch the metal and make them "dull."

Does anyone remember the 'purple' bluing of the past fifty years?

Archie said:

Does anyone remember the 'purple' bluing of the past fifty years?

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I don't know if it was intentional, but I've had some Beretta parts go purple.

Speedo, it's the same as mentioned above. Some alloys react differently. Some folks in the bluing business know about it, probably more - if not all - of them have a reference guide. Chemists would probably be able to explain it in terms of molecular structure and such.

I know enough to pay a specialist to avoid such things.

One of my Dan Wesson 15-2 frames is slowly getting the plum look those guns were known for.

No vinegar involved..

Stay safe.

The purple blueing on Soviet SVT-40's refurrb bolts was notable, and removable with vinegar.

The "B" barrels from Belgium the Finns used on their later M-39's was a deep purple, supposed to be from the amount of nickle the Belgians put in their steel.

I can speak with a little authority on purple and 1911s. There are two things that will make a 1911 part turn purple. On a 1911, safeties and slide stops will often turn purple after a few years. Mainspring housings sometimes do. All of these are cast parts. The steel often has a little silicone in it so as to fill out the mold more easily. Curiously enough, these parts often look just fine when they come out of the tank, nice and black. They turn purple after the passage of time, sometimes weeks or years. And sometimes after only a few days. I have seen safeties that were half purple, half black. The color change isn't always even or consistent. I asked Les about this and he didn't know why. If anybody knows , please share.

The other common reason steel turns purple after blueing is hardness. The harder the steel, the more likely it is to turn purple. If you have ever tried to blue a Garand receiver, you know what I mean. I think Les has his slides treated to RC 40, and for 4140 steel, at least, that seems to be near the cut line. Much higher and your chances for purple go us fast.

caribou said:

supposed to be from the amount of nickle the Belgians put in their steel.

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Sounds like a reasonable explanation. We tried to blue a 1917 Enfield receiver that kept turning plum. 1917 receivers had a relatively high nickel content.