[Michael B]

You could give a try to this:

PHP Code:

new LockWeapon[MAX_PLAYERS];
new LockOn[MAX_PLAYERS];
public OnPlayerConnect(playerid)
{
    LockOn[playerid] = 0;
    return 1;
}
public OnPlayerUpdate(playerid)
{
    if(LockOn[playerid] == 1)
    {
        if(GetPlayerWeapon(playerid) != LockWeapon[playerid]) SetPlayerArmedWeapon(playerid, LockWeapon[playerid]);
    }
    return 1;
}
stock LockPlayerWeapon(playerid, weaponid)
{
    LockOn[playerid] = 1;
    LockWeapon[playerid] = weaponid;
    return 1;
}
stock UnlockPlayerWeapon(playerid)
{
    LockOn[playerid] = 0;
    LockWeapon[playerid] = -1;
} 


Use LockPlayerWeapon(playerid, weaponid) to lock the weapon.
Use UnlockPlayerWeapon(playerid) unlock the weapon.