I also added in slings for the barbarian race I've been working on.
Like GreatWyrm said, you must stick to existing skills. For slings I chose the skill BOW
Also the attack token needs to be specified as BLUNT or EDGE.
Here is what I used for slings, IIRC.
[ITEM_WEAPON:ITEM_WEAPON_BRB_SLING]
[NAME:sling:slings]
[SIZE:100]
[SKILL:WHIP]
[RANGED:BOW:BULLET]
[SHOOT_FORCE:1000] <--- Same as bow, ammo will determine difference
[SHOOT_MAXVEL:1000] <--- Same as bow, ammo will determine difference
[TWO_HANDED:0]
[MINIMUM_SIZE:20000]
[MATERIAL_SIZE:1]
[ATTACK:BLUNT:30:20:smack:smacks:NO_SUB:1500]
Then make an ammo reaction, either set it to be in the craftsdwarfshop, or you could create a custom building for all of your new slings, sling bullets, and any other items you decide to add in. Don't forget to create an ITEM_AMMO entry for your sling bullets. I gave bullets larger item size and similar velocity as arrows (makes them hit with greater kinetic energy), but upped the contact area and reduced the penetration depth (makes them less likely to punch through armor or reach deep organs).
The reaction to make the ammo would be much like you described.
[REACTION:MAKE_SLING_BULLETS]
[NAME:make sling bullets (from stone)]
[BUILDING:CRAFTSMAN:CUSTOM_SHIFT_B] <--- the craftsdwarf workshop
[REAGENT:A:BOULDER:NO_SUBTYPE:NONE:NONE][WORTHLESS_STONE_ONLY] <--- will use any boulder, including sand or clay if you traded for them.
[PRODUCT:100:10:ITEM_AMMO:ITEM_AMMO_BULLET:GET_MATERIAL_FROM_REAGENT:A:NONE] <--- assumes sling bullet item file has item title of ITEM_AMMO_BULLET. Makes ten bullets.
[SKILL:STONECRAFT] <--- dwarfs with Stonecrafter labor enabled will make your sling stones. You can remove the skill line so any dwarf an make your sling bullets, but the bullets will never have a quality then.
A reaction to make the sling would be similar, except use CHAIN:NONE (replaces BOULDER:NO_SUBTYPE) as your reagent item if you want to make slings from ropes (and metal chains!). This would preclude leather slings unless you also add in a reaction to make ropes from leather. Otherwise you can use the THREAD:NONE or CLOTH:NONE item tokens in the reagent line to make silk or cloth slings, and a separate reaction using SKIN_TANNED:NONE to make leather slings.
examples:
[REACTION:MAKE_SLING_CHAIN]
[NAME:make sling (from rope or chain]
[BUILDING:CRAFTSMAN:CUSTOM_SHIFT_B]
[REAGENT:A:1:CHAIN:NONE:NONE:NONE] <--- Will use any rope or chain you have to make the sling
[PRODUCT:100:1:ITEM_WEAPON:ITEM_WEAPON_SLING:GET_MATERIAL_FROM_REAGENT:A:NONE] <--- Assumes the Item file for the sling has title of ITEM_WEAPON_SLING
[SKILL:BOWYER] <--- used bow maker for skill, but could be changed to whatever you like
[REACTION:MAKE_SLING_LEATHER]
[NAME:make sling (from leather]
[BUILDING:CRAFTSMAN:CUSTOM_SHIFT_B]
[REAGENT:A:1:SKIN_TANNED:NONE:NONE:NONE] <--- Will use a piece of leather to make the sling
[PRODUCT:100:1:ITEM_WEAPON:ITEM_WEAPON_SLING:GET_MATERIAL_FROM_REAGENT:A:NONE] <--- Assumes the Item file for the sling has title of ITEM_WEAPON_SLING
[SKILL:LEATHERWORK] <--- used leatherworker for skill, but could be changed to whatever you like
[REACTION:MAKE_SLING_CLOTH]
[NAME:make sling (from cloth]
[BUILDING:CRAFTSMAN:CUSTOM_SHIFT_B]
[REAGENT:A:1:CLOTH:NONE:NONE:NONE] <--- Will use any cloth laying around to make the sling
[PRODUCT:100:1:ITEM_WEAPON:ITEM_WEAPON_SLING:GET_MATERIAL_FROM_REAGENT:A:NONE] <--- Assumes the Item file for the sling has title of ITEM_WEAPON_SLING
[SKILL:WEAVING] <--- used weaver for skill, but could be changed to whatever you like