Send the request event to the client, get the data, then bounce back to the server with the data. Use these natives;
mp.game.invoke("0x8DECB02F88F428BC", mp.players.local.handle, mp.game.joaat("weapon_SpecialCarbine"), 0); //Check specific weapon
mp.game.invoke("0x015A522136D7F951", mp.players.local.handle, mp.game.joaat("weapon_SpecialCarbine")); //Check ammo count on weapon