Well:
FT inferno is 12 TU for 180 fire dmg. Once within 8 squares it won`t miss.
SMG - 14 TU for burst, 125 dmg. Reasonably accurate, but to low dmg, and 2 TUs more than inferno. 20 TU for full auto, 375 dmg. Target needs to be within 4-5 squares to catch most bullets. Good dmg, but it`s 20 TU vs 12 for inferno...
Shotgun with flechette - can`t recall the exact cost, bu it`s over 14 TU for 3 round burst, ~260 dmg. Must be really close (3 squares recomended), flechettes spread a lot. But sure, good dmg, however TU cost is significantly higher than FT`s inferno. Full auto - 27 (?) TU, over 600 dmg. Spreads even more, and is VERY costly TU wise. Also it empties the magazine. Dmg is over the top, but that`s it.
Obviously FT is useless outside the 8 squares range. But over this distance, IMHO assault rifle starts to beat other close combat guns, due to it`s high accuracy even on full auto.
Now, take into account, that armors usually have much higher resistance (IIRC about 20 protection more) to bullets, than to fire.
Of course it looks different on various difficulty levels. At certain point these 180 dmg from FT just doesn`t cut it, and flechette shotgun will reign supreme.
In above comparison I didn`t take into account weapons, that you can discover. Quite likely you will get bolter rifle early on, and it`s a strong competition for close range fights. Plasma is even better. But then, these hi-tech toys are supposed to be better, no?