Don't do it.
Douglas muzzle loading barrels were heavily cold drawn to octagon shape, from round bar.
They have very high residual stress in them. Unless you got the nice, black 7-groove barrels from Golden Age years ago, those were stress relieved. That is why they were black.
With enough care to take off the same small amount from each opposite side while swamping the thing might stay straight.
But there is one more dimension to consider. That is bore diameter.
The Douglas barrels which I have examined had high tensile stress outside, balanced by equally high compressive stress in the bore. As you remove metal from the outside, you are removing metal with residual tensile stresses.
Tensile and compressive stresses WILL balance, or the thing will wander across the room.
So the compressive stresses in the bore will reduce.
Huh?
The diameter of the bore will change. I believe it will become larger.
Anyway the thing will never shoot well, no matter what you do.
Sincerely, Your Forum P.I.T.A. metallurgist, Jim Kelly