So I was looking at Arsenal, not having the SR4(A) edition and just the original SR4 book after the run we had tonight and I think we screwed up just a touch when it comes to the rules for flechette damage.

Looking in Arsenal it lists several shotguns and the frag grenade as 'flechette' damage but only gives an AP modifier of +2. This leads me to believe that it is only the special 'Flechette' rounds that have the +5 AP modifier. This would make sense in many ways.

Shotguns are designed around that type of ammunition. They tend to have longer barrels with a smooth bore and a much larger cartridge for more gunpowder. Most other firearm barrels are scored to help spin the bullet for trajectory and accuracy. They also tend to be shorter than a shotguns barrels and thus not optimized to use Flechette ammo properly which would explain why they get a +5 to AP over a shotguns +2.

This would also hold true for Fragmentation grenades since the power of the explosion propels the fragments stronger than simple cartridge sized charges.

Anyways, that's my take on it, that the +5 AP for flechette ammo is assuming it is used in a weapon not designed for flechette rounds, and how I will be running it myself in the future. Unless of course Arsenal has been altered by errata that says shotguns get +5 AP modifier and Fragmentation grenades do as well.

SR4A fixes it.  In SR4A, all flechettes, including grenades and shotguns, are listed as AP +5.  That means increased choke shotguns also do +7 and +9.

Wow, that makes them downright useless.

Not at all.  First of all, unarmored people don't get any +5, they get ripped in half.  Good against critters, many of them have no armor rating.  Also, on average to resist +2 DV, you need 6 extra dice.  The ammo only gives you 5.  So while an armored target might resist the extra damage without a problem, they do just a bit worse than break even.  That makes it better than regular ammo in all cases.  Not the most amazing ammo, but not quite useless.  And shotgun flechettes get added benefits of spread which can make them pretty insane on burst fire or full auto.

Guess it depends on how you look at it. Not everyone is as armored as Cobalt or Hellborn after all.