At the hardware store, it's brick cleaner or toilet bowl cleaner. Ask them where the smallest package is, you don't want the gallon size I guess.
And it's hydrochloric acid - about 30% I think - so use it outside, rinse really well, oil the gun immediately.
I've read of folks using it indoors to clean tile floors and having the vapor discolor every stainless steel appliance in the room.
Have you tried a lead away cloth or maybe blood. I hear that works.
John
edited to add: The directions for cleaning brick is to water it down something like 10- or 20 to 1, but read the instructions. If I was going to use it I think I'd mix a weak solution in a plastic bucket, dunk the gun and then either dunk it in a bucket of clean water or hose it off two or three times. Repeat as necessary. And then dunk it in oil.