I've had some issues using express type bluing solutions in the past on both Model-21 barrels and a couple of others as well.

The problems I've experienced were areas that looked translucent/thin, brownish or reddish especially on the breech end of the barrels. I called & talked to Mark Lee about these issues and he told me that the first application of the solution is the most critical and the barrels need to be evenly warmed before the solution was applied. He said that after you do your first boil to be sure to examine the barrels under a bright light or take them out in the sunshine and if there are any areas that are thin, translucent, off color or bright metal to polish those areas back down to bright metal with either 320 grit sand paper or green scotchbrite & then apply the solution to those areas again & re-boil.

He said if you don't take care of these problem areas after the first boil, they will only get worse with the more applications & cycles you make.

I've found that for me these express solutions are great for small parts, but I never did get the hang of getting a set of barrels done right with them. I started using Laurel Mountain forge browning solution and doing barrels the old-fashioned slow rust process described in their instructions and have had great results ever since.

My opinion is the express solutions have a learning curve to them and an experienced person who uses them on a regular basis will learn the work arounds for problem areas as they occur. As a part timer hobbyist. I feel that the slow rust process using LMF is much more forgiving and easier to use in the long run.