Great Question . . .
We had that same discussion not that long ago and didn't get very far with our small collection.
So I'm eager to hear from others in the museum world
I've already had long chats with my Historical Reenactors friends on this.
In the mean time, here is what I know based notes in the file and those conversations
Firearms require routine maintenance for preservation and safety.
The metals are prone to corrosion especially when not used regularly and the oil retards it.
Also any firearm which uses black powder MUST be cleaned and verified for safety reasons - its volatile!
Same holds true for powder horn - you really do need to get rid of as much of the powder as possible.
Cleaning the horn is another issue that I clean to conservators and those in the know.
Not sure if there are any conservators out there who actually do this work . . .
We've called on the local police to assist with the black powder in the past.
Your volunteer might be qualified to do the work - ask for a resume / past experience.
Also, someone has to document everything noting what was done to the firearm and any issues like corrosion.
I know our firearms were cleaned and oil back in the day . . . but a 3 word note just isn't enough.
Hope this help. As I said, hoping to hear more on the topic.
Chrisso Boulis
Registrar Records
Penn Museum
T: 215-898-4088