I've tested with this:
fm_set_user_bpammo(id, weapon, ammo + xxx[id] * xxx);
Somehow ,but BACKPACKAMMO won't be increased(when holding current's weapon ammo is increased) , but it's good for me ;)
OK will use your code. And I wanna ask next question about increasing ->30<- | 90 ,ammo in clip,possible to increase?