Instead of setting a variable use /var, that way its local to the script and not a global variable. maybe something like:

on 1:TEXT: !briefcase:#:{
  var %x $rand(1,3), %m
  if (%x == 1) %m = pulls a pistol out of a briefcase and shoots $nick in the head
  elseif (%x == 2) %m = pulls a pellet gun out of a briefcase and shoots $nick in the chest
  else %m = pulls out a huge wad of money and tosses it to $nick
  describe $chan %m
}