The Slipslinger Style feat from the Weapon Master's Handbook requires you to have the Warslinger halfling racial trait. However, right now even if you take it you still don't qualify for the feat.
Weapon Master's Handbook is kind of a mess at the moment. I'm slowly fixing it, but it requires alterations to the code of almost all the hard-cover books and many of the soft-cover books. I will try to address this specific feat so it works in the next update, though.
I think I may have a fix for this: Change 'TYPE.Warslinger' to 'Halfling ~ Warslinger' in the PRABILITY tag. The same will need doing for the other two Slipslinger style feats.
Where do we stand with this team?