I am no expert but I have not used any copper sulphate compounds in my browning since my early trials.
I found that it cut into steel as well as iron which could 'muddy' the final contrast. It also tended to promote after rusting which has not been a problem with the boric acid etch that I use now.
Another problem with copper sulphate is that it precipitates copper deep into pits and engraving which can be the very devil to remove. In one particularly bad case, I had to use a concentrated nitric acid bath to remove the copper which made a mess of the base metal as well. The air was blue after that event!
Yet another problem is that unless all the jointing areas are masked with varnish or paint, if a full immersion is used, the copper sulphate etch can actually loosen the jointing of a border line jointed gun to the extent of rendering it loose. Same can be said of using Ferric Chloride in a full immersion.
This can be avoided by applying the concentrated solution by cotton pad only to those areas that you wish to etch but this is tricky and very difficult to keep off breech ends etc.
This is not to diminish the great results that have been achieved by Cadet and all others that use copper salts for their etching, it is just beyond my modest abilities.
