It's my experience that with a muzzleloader (and a handgun) then heavier bullets will actually strike hogher than lighter ones...

A heavier bullet recoils more, throwing the bullet higher, with the slower velocities in muzzleloaders and handguns it makes more of a difference than in centerfires...

Now...Although you say you've lowered the rear sight, you could also raise the front sight and do the same, even if it means replacing it and filing down to fit...

What Hawken do you have??? How is the sight attached to the barrel???