Guns and ammo rules clarification, SR4A flechette stats are confusing Options

[FriendoftheDork]

Hello everyone, I'm and old SR GM, and some may remember me from before on these forums.

We have started a new game, this time using SR4A rules. Now after forgetting my old house rules and the erratas I'm looking at the SR4 anniversary book, where I suppose the old errata has been applied to.

Flechette: It says it gives +5 to the targets Impact armor, but both the Viper Slivergun is listed with +2 (metal slivers that count as flechette), as is the Remington 990s shotgun (w standard flechette ammo), while the Remington Roomsweeper w/flechettes (actually said to be buckshot) has +5AP. Now are these different ammunition types or are there not? If they were consistently wrong (as in copy paste from SR4) I could understand it, but this is confusing.

There are no rules which weapons can use which type of ammunition, so I'm assuming everything can use everything unless specifically noted (some can use only x). With that in mind, why would anyone ever buy a shotgun (remington 990) to fire the expensive and almost useless flechette ammo, when for example the Ruger 100 sporting rifle can do the job at much better range? Or why get the Mossberg CMDT when an assault rifle can fire flechette ammo full auto while retaining superior range, no extra recoil modifiers, and much better ammo capacity?

That's it for now.