[Calgon]

I've written some code below which should help explain how it can be done. See the comments for an explanation.

pawn Код:

// Create a variable to handle the ID of the pickup
new
    iWeaponPickup;

public OnGameModeInit() {
    // Create the pickup with the AK-47 model (you will have to change this if you want different weapons)
    // You can find a model list here: https://sampwiki.blast.hk/wiki/Pickup_IDs
    iWeaponPickup = CreatePickup(355, 2, 0.0, 0.0, 9.0);
    return 1;
}

public OnPlayerPickUpPickup(playerid, pickupid) {
    // Use the iWeaponPickup variable to TRACK the ID of the pickup
    if(pickupid == iWeaponPickup) {
        // Create a variable to store the information we get from GetPlayerWeaponData
        new
            iWeaponData[2]; // 0 - Weapon ID, 1 - Weapon Ammo

        // Check weapon slot 5 (which holds the AK-47) for the weapon ID and the ammo amount
        // Weapon Slot List: https://sampwiki.blast.hk/wiki/Weapons
        GetPlayerWeaponData(playerid, 5, iWeaponData[0], iWeaponData[1]);

        // Check if the weapon in the slot specified is the AK-47, if so, add 100 to their ammo
        if(iWeaponData[0] == WEAPON_AK47)
            SetPlayerAmmo(playerid, WEAPON_AK47, iWeaponData[1]+20);
    }
    return 1;
}

As for hiding player weapons and still making them usable, I'm not quite sure that's possible but it might be possible to hide player objects over the weapon.