Any thoughts on using the old salt/vinegar/peroxide formula on a non stainless gun barrel?

Would embrittlement actually be a concern on this?

I don't understand why embrittlement would be a concern.  Rust bluing has been around a long time, and I haven't heard of any problems before.  Maybe I don't understand the question properly, however.

OK, so I asked my favorite AI girlfriend if hydrogen embrittlement was a problem when rust bluing with hydrogen peroxide, and she said it was a potential problem.  So if it concerns you, you could always use one of the traditional nitric acid based rust bluing solutions.  Probably easiest to just obtain one of the proprietary solutions readily available, as it is difficult to obtain small quantities of nitric acid.
They are expensive however.  Seems like they went way up since I bought my last bottle.  Just another example of transitory inflation, I suppose.  Supposedly, heating to 300F will remove some of the hydrogen.  Usually, steel will not be affected by that low of a temperature, but it might be if you went higher.  What exactly are you trying to rust blue?  That might be important.

H₂O₂ only has an additional oxygen atom from water.  I don't think any more hydrogen will diffuse into the steel than any water/acids will.

Picked up a (presumed 4140) rifle barrel in the white ,30 odd years ago.  I'm thinking about rebarrelling a shot out one with it.

Pretty much a weekend boredom project. Nothing critical but I also dont want it crack 10 years down the road due to chemistry issues.

A) hydrogen embrittlement shouldn't be an issue.

B) If you're that worried, just bake it out like professional hard chrome platers do.

c) There's always molten salt bath bluing; melt potassium nitrate, dip parts.  Cheap.  Looks great.

I would go right ahead.  I can't see a problem with that.  The interior of the barrel is where strength is required, and that doesn't even come in contact with the rusting solution.