I was actually just watching some youtube videos in which it was shown that swords can theoretically cut bullets in two mid flight, though the path of the fragments seemed rather erratic, sometimes deflecting outward at an angle and other times simple travelling on a similar path as two pieces.

Whether the bullet was deflected or cut, however, seems rather moot to me, as the true impossibility lies in turning one's body around in mid air in the time it takes a musket ball to travel six feet.

Data varies on how fast a musket ball from a flintlock pistol will travel, but I found data ranging from a muzzle velocity of 700 fps to as high as 1500 fps depending on the composition of the black powder and the particularities of the gun used.  Even using the lowest number of 700 fps, the time that it would take the musket ball to reach Guardian at a range of six feet is approximately 0.00857 seconds, or less than one hundredth of a second.

Even if Guardian were somehow able to turn himself around in the air without having used the proper leverage when leaving the ground, it seems impossible to do so in such a tiny fraction of a second, no matter how agile he is or how good his reflexes are.  Even more difficult would be locating, tracking, and deflecting the flying musket ball after doing so, all within less than 1/100 of a second.  Not trying to be a jerk to anyone here, but this act seems physically impossible to me, even had Guardian possessed superhuman speed to go with his agility and reflexes.