I have browned alot of barrels and iron hardwair for guns.
The best method I found is to degreese the part you want to brown with a good degreeser from a local auto parts store. Now useing a
Q-tip apply your urine to the part. Every couple of days apply more urine to the barrel
or iron part, untill you get the amount of rust or brown finish you want. when you are satisfyed with the finish apply oil to the part to retard the rust. Easy cheep and it works. Old Snow

I used the plum brown, on a GPR , and loved it, I set the gun a rest and heated it up with a propane torch, it worked GREAT !, get it really hot, and touch the brown on with a rag or what ever,it should sizzle when it touches it. I did it about 5 times , one area at a time, then wiped it off and oiled. looks great.

Track of the wolf sells a couple of good browning products. The best of the to is under government restrictions right now and they are having problems re stocking it.
The second is made by Laurel mountain forge. I have used it on a coupe rifles and it works rather well if you follow the instructions. It’s a degreaser and brown all in one, the cost is not bad either if I recall 4 or 5 US dollars per bottle.
While pissing on you barrel will also brown it " I have done this as well" I do not like the pitting of the steal that it leaves
And lets face it some of us pee strait ACID LMAO