Playing Ao100, dlevel24, Ammochain build + going for brute/berserker just in case I find a DS.
The following happened when chainfiring my Plasmarifle A3P1:
#.####
#....2
#@...#
1....#
#....#
I fired and killed a monster in position 1 (a doorway, if that matters). Chainfire autotargetted the monster in position 2(also a doorway!), and I hit f to fire again.
As the barrel swiveled around, the shots not lost to the turn landed somewhere around here:
#.##X#
#....2
#@...X
%....#
#....#
I fired again, and received a lot of "Boom!", my shots seeming to alternate between the two points. Thankfully, after my third volley, a monster appeared in the top doorway, so I switched target to him, and chainfire successfully autotargetted monster #2 this time around.