This seems odd, the ball would not touch the riflings and become staple unless there was a patch or the ball was larger than .360 in diameter...
If the bore was .360 then a .362 diameter round ball would engage the riflings and be somewhat stable in flight.
How far away was the targets that he was shooting?
25 yards?
50 yards?
100 yards?
It sounds like when the barrel was dirty, it became a "smoothbore" and the foulings would then act as his patch, so to speak, but a dirty barrel will not spin the ball for stable flight.
So this lead me to think that the shooting distance was short, around 25 yards or so, no time for the bullet to stray far from the line of sight...
Maybe this is one of those, "ONE IN A MILLION" guns that defies the laws of physics, it throws out the paradox of the patched round ball for a more mystical reason...
There's one other thought I had on this...
What if your friend was pulling your leg?