About javelins, I don't like the traditional way to portrait pilum-like weapons (adding the AP attribute), because this make them more effective against armored enemies than against unarmored ones : but historically the purpose of the metallic javelin was to
penetrate the shield, and hit the unprotected flesh behind it, not to pierce armor! It's the contrary, the armor is the only valid defence against them! Since the AP attribute don't affect shields, it makes AP javelins perform in an unrealistic way, and so it should be discarded IMHO, and replaced simply by a raise of the attack value. If you agree I'll change AP javs accordingly, I'm waiting for your opinion
About slings well, the debate is open... According to
http://www.slinging.org/ and the EB team they should be more effective and have a longer range than bows, particularly western ones, but the bows have the advantage that they can be used in quite tight formations, when the sling need a lot of space, and the bowmen have actually
more ammunition than the slinger... In war slingers usually used common stones only in emergency situation, normally they carried their projectiles, that could be simple river-stones, or made of pottery or metal, and obvioulsy similar ammos take more space in a bag than an arrow...