I, and most all other gunsmiths and military armorers have used them for many years.
It's actually a paint in a stick form.
You scrape the end to expose fresh material and rub it in the markings until they're full.
Remove the excess with mineral oil or a dab of lacquer thinner on a paper towel.
Let dry for 30 minutes and apply lube to prevent the metal from rusting.
It lasts just about forever, but can be easily removed with lacquer thinner and a toothbrush.
As long as you don't scrub around the filled letters with a brush, rub with a rag, or spill a solvent OTHER THAN bore solvent, it's stays put.
Bore solvents don't seem to attack it that fast.