First, the terrorist needs to demonstrate its lethal, or at least the weapon is convincing, so a 1st shot must be initiated. Otherwise he will get laughed at.
Terrorist aint stupid, with that "rice gun", even he managed to kill a person on the 1st shot, assuming an instant & deadly penetration into the heart or critical brain part, how do you expect the threatened crowd will react after the 1st shot while watching he is busy re-loading the "rice" ?
I don't think its good enough to make such a dangerous threat that will convince the whole plane passengers.
Not a security expert, but I don't think that rice gun is more threatening than a metal fork.
A weapon for assassin, probably yes, but with big chance of failure, a weapon for hijacking a plane ? C'mon.