I hear the contempt for 3RD burst but I would think a greater mind than mine had some good reasons for it...
Suppose you are in close quarter combat; room to room; you enter a room with a 285 lb. , 6'4" enemy wearing multiple layers of clothes and pointing a shotgun at you...do you raise up to get your sight picture, shoot one of your 9mm or .223 and wait to see the reaction? Wouldn't a 3 RD. burst to some vitals be to an advantage? Suppose he is high on something and won't go down too easily...2 three round bursts ? It is pretty quick and sure ...you sure don't want reaction time on his part if his shotgun is pointing your way...
FA definitely has psychological advantage to the shooter if confronted with multiple enemies...
The popular theme with our high tech mentality has found criticism with the FA and burst capability...popularity dictated the demise for the .45 and we all went to the 9mm...now we have to shoot 17 rounds to accomplish what 3 rounds did before...
If I confont an armed intruder who is attacking me, whether I have a MP5 to burst him or whether I have the 92FS or a 1911, he is getting three rounds instinctively before I stop to evaluate whether I need more or not...It is just easier if the 3 rounds only require one sight picture and not three...