Is it certain that its original purpose was for a rifle? It might be for a fishing rod or a musical instrument? Flag pole? Bow? Spare peg leg?
I found two great long canvass cases things at a yard sale for one dollar.
The one that I use for a musket cover has a NATO stock number on it which identifies it as an army radio antenna case.
The other was wierd, I gave away it to a buddy who recognised it as a cover shaped to fit his banjo.