Basically how you view each guy and how it should be and when it should be used. Now it should be noted typically you want to be aggressive with aliens to deny any get away with that...
Buck shotgun- best all around but horrible versus stuns and long abilities, the workhorse of delta and scourge of lurkers and runners. It heavy counters them though in truth buck is best with a robust group able to quick react to jumps.
Slug- great against tier 1 and tier 2(excluding carrier+hivelord) for the little stun it offer at range, not much else a true utility weapon
Flechette- Armor shredder and can pack a punch, the long range buck more or less expect no ministun. Best verus armored tier 1, 2, and queen as they have horrible armor regen in the first place. Not often seen as people going shotgun are typically going for utility(slug) or raw kill power(buck)
41- if you can hit reliably something and keep the bullets coming the damage can be obscene, issue is hitting. Fantastic in long hallways and against range castes.
39- also nasty if it can hit but a more close range cousin of the 41
42 is just too slow to kill much but still liked for being a discount harassment tool against boilers and people resisting fire in the open but it low burst potential makes it weak vs harassers
Flamer- useful if you got a MD to track and finish or if you use to chase the burning alien otherwise it denys alien movement or helps get a lucky kill through it DoT. Typically seen cooking delta
Pistols- Stupidly high damage and stupidly fast, never knock the sidearms. Not always taken due to lack of slots, IE it is taken by shotgun ammo slot or medical/ammo on pockets though shoulder holster does increase the rate it is seen. Not often used as people get absolutely wrecked before they get a need or a chance to use it.
Add anything else you think about the various guns
EDI:fixing mistakes adding stuff