I would have to agree on the necessity of training. During a stressful, adrenaline-pumped situation when you lose all thought processes and motor skills, you only do what you have trained for. Statistics support that most firearm owners don't train for situations with their gun, let alone with a bayonet on the weapon.
A good alternative would be a nice quick, release blade in the pocket. When the shells are spent and you have virtually reached hand-to-hand combat, it can be out in an instant. Moment of surprise can lead to a quick slash to throat, wrist, thigh etc.