I keep looking at this but every time I do, I find myself puzzled. I have to say that I don't think it looks like Greek (at least, not any Greek script I've ever seen - I wouldn't pretend even for a second that I have any ability in the language), or indeed Cyrillic.
Weirdly, the marks almost look more like the sort of talismanic stuff you normally see on swords and the like from Northern Africa, to my (untrained) eye at least. Perhaps a North African pistol barrel?
Also, what's up with that little scab of metal toward the fore-end? It looks as if someone's been at it with a file.
Best,
Meredydd