If you simply want to dull the finish:
Remove the barrel from the stock. Wipe down really well and allow to dry. Wipe down with a solution of lemon juice and water (50/50) and let stand for an hour.
Wipe down with a hot, wet rag, allow to dry. Repeat.
After 4-6 cycles, it'll start to develop an nice "French grey" patina. After that point, wipe down with a hot wet rag again, and after it dries, give it a heavy coat of oil.
Then take it out and use it. The grey will darken as you use it.
Mario