Birchwood Casey or Oxpho gun bluing. If you only go with a couple of coats, and then neutralize, it will come out grey. Add coats to get what you want. If you go too deep, rub it back (carding) with a wet terry rag.
Get it where you want it, rub it back once more with rag and water, then oil it to stop the process.
When applying, do long even single sweeps. DO NOT re-coat where already done until dry and carded. It will cause reactions, and cause streaking and multiple colors, like browns, coppers and yellows. One sweep of each flat, let dry and card.
Now and then later on (days, weeks, etc...) you might get a start of rust spots. Just a bit of fine steel wool and oil will rub them out. Just not too hard or you will mess up the blue/grey.
If you don't like it, do it again! That's part of the fun. There is no right color, only right for you!
Birchwood Casey makes a bluing kit. Has Perma Blue, Bluing/browning remover and Degreaser. Plus swabs, steel wool and emery cloth. And directions. Plus, search Birchwood Casey barrel bluing on UTube. All sorts of experts!
Then next time, get a bottle of BC plum brown! and Break out the heat gun