It's called a keyhole, it happens when the bullet does not get adequate spin from the rifling in the barrel and the bullet tumbles through the air as a result.
Can result from a number of things, less than ideal rate of twist, lower than needed velocity, bullets not quite tight enough in the barrel to be adequately rotated by the rifling, or enough fouling in the barrel that the rifling loses its effectiveness.
Was this the ammo you normally use? How does the bullet weight and velocity compare to ammo that does not keyhole? Taken a good look at the rifling in the barrel and given it a good cleaning anyways?