I still think that it could be curved...maybe if the inertia of the loaded gun was going in the opposite direction of the groves...?

And that bullets naturally should curve over long distances.

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The lights always on here. ;)

No.

From another external force? Possible.

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"I give the probability of this working at zero" - Dan DeMatteo

Imparting any other velocity on a bullet (or any object in motion) will result in the same thing so yes you can cause a bullet to curve by moving the barrel while you shoot.  Although this will almost negligible movement in that vector compared to the force imparted by the propellant.  Thus it isn't going to "curve" around a target because one did that.