Flash rust is still rust. Does it matter if you destroy the barrel with a thousand cuts, or just one?? By using tepid water, you avoid the problem all together. The water dissolves the salts. The soap is there to emulsify the acids, the salts, and the carbon, both from the residue from the granules, and from the graphite coating of the granules. Flushing the barrel a couple of times removes then all.
If you insist on using boiling water, than be a bit calculated about how you use it. Try using it initially, and then follow up with tepid water while the barrel is still wet to cool both the water and the barrel. Cooling that barrel down to room temp is what we are after to avoid the rust.
I used to use boiling water, then as hot as the water came out of the tap, to clean my gun barrels. I burned my fingers and hands many times. I found it very difficult to get the water down my small barrel without some spillage each time I cleaned. And, I always had flash rust.
Someone at the club told me to use tepid water, and went through the various chemicals and elements that comprise the residue we are cleaning out of the barrel. Carbon is the largest component. Sulfur, and Nitrates creates the acids by combining with moisture in the air. The salts come from compounds derived from these elements that are not used to make the acids.
I have not had to deal with flash rust since.
Advice here is free. Use it as you wish, or ignore it. No one is forcing you to do anything. If you think we are wrong, do your own thing. Its your gun. If you ruin it, you have only yourself to blame, and to look to for repairs, or replacement. NO drill sergeants, here! :thumbsup: