Does the superior slinging only work for THE sling or for all slings?
Especially the halfling-staff sling would be a good candidate, too.
Superior Slinging (Ex)
A halfling sling master can reload her sling as a free action, similar to the way an archer can nock an arrow. This ability counts as the Rapid Reload feat for the purposes of meeting prerequisites for other feats and prestige classes. In addition, only half the weight of the sling bullets she carries counts towards her encumbrance total; she is adept at positioning her bullets in such a way that makes them easier to carry.
Edit:
Another question: The slingmaster's list of bonus feats includes manyshot which can only be used with bows. Is that intentional or should the slingmaster be able to use it with slings as well?