Rich,
I have use the Birchwood Casey Plum Brown on locks, and barrels. Smaller parts are easier to do that the barrels but with a little care you can get a nice finish with the product. the key is getting the parts totallyclean of all oils. I use real hot water and dish detergent first and then lacquer thinner as a second wash.
You heat the part up until water sizzles on the part, just like a pancake griddle and then apply the solution. I use one of those little foam brushes you can get for 49 cents at the hardware store. Once coated give it a second coat. Then drop the part in hot water and throughly wash off any remaining reagent and surface scum. I then reheat the parts and coat them with WD40 to dry them out and then bee's wax to protect the finish.
If you want black, as for rear sights, drop the browned part in a pot of boiling water and in a few minutes you have a nice rust blue/black part.
Best Regards,
JMC