Whip, you could put some screws into a small kitchen sieve and try that first, though it takes larger parts to really get a sense of what the colors are like. It requires quite a bit of nitrate, since melting the stuff is rather like melting snow down into water, there are probably cheaper sources of KNO3 online, I just go to Lowes for mine. The only durability problem so far for me has been with the grip frame of a Pieta '61 Colt on which the nitre bluing wasn't as durable as the factory bluing, I'm going to try brass plating on it using the method Richard suggests. My guess is that it will take a while to get enough nitrate melted to blue large parts on a grill burner, and that heat rising around the vessel which contains the molten nitrate will make lowering and raising parts into the bluing bath a bit tricky. I made my melting vessel with a rim which sits on some insulating fire bricks; makes it safer to work above, and heats more quickly. Being the impatient sort, I help my burner accomplish the initial melt using a propane torch. Once you get the nitrate melted the grill burner would probably do a fine job of keeping the salt melted and at the right temperature. You'll need some sort of frame above your melting tank so that you can suspend the parts in the solution, you don't want them touching each other or the bottom or sides of the melting vessel, I use a steel rod above the tank as a frame, and hang the parts from it with black iron tie wire.
