Which weapon you replace? SG-550? it has ammo id 4. And secondary ammod id / amount should be -1.
If you want to change ammo id, you need to hook Item_PrimaryAmmoIndex and override ammo and index and hook ItemInfo (only latest amxx) to override ammo there too. I'd suggest you to replace g3sg1, since it has ammo ID 2.
Cheers.