Mauser
Mr Martin's secret formula...... you had the general idea, here's how it works.
First of all, pretty labor intensive, AND, you'll need a sweat box, or the like for humidity.
Anyway, brown without carding, till way beyond where you would normally go. Reapply solution every 6 to 12 hours.
Here's the hard part....., sand all the browning off, leaving the pits.
Cold blue everything, then rub back with Skotchbrite pad and your favorite preservative. I use Breakfree CLP.
Gives a nice patina.
I should add, ..... round off the edges of the barrel flats. Looks silly to have an aged patina, on a crisp edged barrel.