Hey saw you're trying to remove the shine on the musket you've got. I figured I'd let you know my method for the matchlock I have.
I removed the barrel from the gun.
Plugged the front with a synthetic wine cork. What caliber is the gun?
Plugged the touch hole with a toothpick. If you haven't drilled the touch hole already, even better.
Then got a tube of PVC in a dimeter and length that could completely submerge the barrel and a cap for one end.
Buy store brand tomato juice. Mine was Piggly Wiggly brand.
Sand blast or scotch brite(green) the entire barrel, making sure to have all the lines run the length of the barrel if you go the scotch brite route.
Wipe the entire barrel down with acetone or rubbing alcohol and a clean rag. Acetone will be better.
I didn't do this part but I recommend doing it, seal the threads at the breech with clay or wax. Just enough to fit in the gap of the breech plug to barrel. Clay probably works best.
Then, you might test the seal of your pick and cork by submerging the barrel for 20 minutes in water. If it gets any in the bore, you know you have a leak somewhere.
After confirming barrel is sealed, fill up the tube with the tomato juice and submerge your barrel in it for 20 minutes.
Remove for 5 minutes.
Repeat until desired gray color is achieved.
Rinse thoroughly with water.
Then make a baking soda paste and rub down the barrel with it and a toothbrush.
Rinse with water.
Repeat until you don't see the paste react anymore.
Then oil the barrel. The longer the time from baking soda rinse to oil, the more brown the color will change to. Realize this will promote rust in the future(similar to how browning solution encourages rust) so grease on the underside of the barrel and a good preserving oil, like ballistol, will prevent rust from getting too bad. If it ever gets fine surface rust, wipe it back with a rough cloth, and reapply oil.
Don't be afraid of rust as long as you keep oil on it, it will not grow.
Biggest thing is to clean the tomato juice out well and enusre that the gun is oiled.
I also scraped the varnish off my stock and gave it an oil finish.