as a gattling gun's barrel rotates, you will have to wath for tennis balls getting caught between the rotating barrel and the loader
as for speeding up the loader you could try a using 2 rotating wheels, as used in tennis ball guns and bowling ball return mechanisms to draw the balls through
like so
_____. ._____
)_____(O)_____(
the wheels would both turn outward and propel the balls into the barrel
|