try the "do commands
http://www.ofpec.com/COMREF/index.php?action=list&game=All&letter=d#doWatchThis would have the gun look straight up.
_x = (getpos _unit) select 0;
_y = (getpos _unit) select 1;
_unit Dowatch [_x, _y, 1000};
This would get the unit to look foward and up 500 meters.
_apos = ( position _unit );
_posa = ( _apos select 0 ) ;
_posb = ( _apos select 1 ) + 500;
_posc = ( _apos select 2 ) + 500;
_cpos = [ _posa, _posb, _posc ];
_unit Do Watch _cpos;
Mind you I haven't actually tested it but I belive it would work.
i am not sure exactl what you are trying to accomplish if you were more specific you could probably get abetter answer.