How about unbreeching the barrel, and running a string down, pulling it tight and sighting down the string? I had a suspected bent barrel once, and that was the advice I was given.
And, here's another, similar method for cartridge barrels, that probably could be adapted for ML's:
If you don't know, you can check a barrel for straight, by looking through it, and observing the light rings. They should appear concentric throughout the length, without running to one side of the barrel.
Decap a cartrige, and insert it into the chamber. This focuses the light entering, and makes the light bars much easier to observe.
Before unbreeching, if you have any doubts, you could experiment on a piece of steel or copper pipe first to see if you think the technique will work.
good luck, SCL