I use a simple sharpie marker in silver color. This generally isn’t in an area that gets a large amount of friction, so there’s not a lot of risk in it rubbing off. For those that really need EVERYTHING to be subdued (you know who you are), I use a red sharpie.Here’s a couple examples: the silver is on my personal weapon, the subdued is on a work rifle.
Witness marks can help by providing a visual indicator when a bolt/fastener comes loose. Witness marks also help you provide the same torque when a torque wrench isn’t available or practical.
I did this a while ago with my duty rifle, PRS guns, and a switch barrel gun I use (LMT MWS)
Editor’s note: Kelly sent this over a while back, but I am so unorganized, I kept dropping it. How? I always want to wait for the “perfect” time to post things. Then it falls through the cracks. Obviously I need to find a better solution. My apologizes to Kelly for the delay. —Marky