You would need more feats than as an archer ,but yes you can make a mean slinger.
Juggle Load is one more feat tax that you would have to swallow.
You wouldnt have to be a halfling although it certainly would be a good choice.
An archery build that uses a halfling, basically? I mean, here are a set of example (post racial) stats for your standard fighter/ ranger/ slayer/ whatever martial archer
STR: 14 DEX: 17 CON: 14 INT: 10 WIS: 12 CHA: 9
With warslinger, the reload is a free action, which basically just turns the sling into a short bow with ever so slightly lower damage dice and range. You also do not have to worry about composite bows and strength ratings and the like. I keep the strength up for that reason- extra bit of damage. Anyway, despite the slight downgrade...you are still an archery build, basically- you are going to do fine for damage.
If we wanted to get very, very tricky with all this though, I suppose we could go juggler bard.
With juggler bard, you are considered to have a free hand as long as you are juggling less than 3 items (there are later increases to this, but this suffices for our purposes). That means that you can have 2 slings, and still be considered as having a free hand to load ammo. Thus, you can TWF with your slings using this archetype.
I know you can do similar things with alchemist...but I chose the bard archetype because it allows for nice beefy bonuses to attack and damage from inspire courage, and then some more from arcane strike. That should be enough for you to enjoy a nice bit of damage.
Oh, example bard stats:
STR: 14 DEX: 16 CON: 14 INT: 10 WIS: 8 CHA: 14
Halfling warpriest is perfectly capable as a slinger.
Actually, with the juggler bard, the need for a ton of extra feats to do both archery and TWF might be a bit too much. Maybe just dip 2 levels so you can juggle, and then go with fighter so you can get a ton of bonus feats.
Slayer and ranger also suffice...but if we are talking about the sheer number and scope of bonus feats....