Okay, this one may be a bit roundabout, but here's an answer that hopefully helps:
If a gun's listing doesn't have (f) after it's damage code, then it is not firing Flechette (or "shot") ammunition. Even teh shotguns ... that 7P for the Remington 990, is based on it firing a solid slug. Basically, a single really big bullet.
if you load Shot rounds into a shotgun, it uses the Flechette rules.
A "Narrow Spread" - where the gun is keeping all the little individual bits pretty close together - does +2 damage, BUT, it's Armor Piercing suffers a +5 penalty. (Important note: +'s are bad when talking about Armor Piercing, -'s are good.) So with a narrow spread, the Remington deals 9p(f), and it has AP +4 (versus Impact, not Ballistic).
When firing a "Medium Spread", teh gun is letting the individual bits of lead spread out a little more. The damage isn't as good, nor is the armor penetration - but it does have benefits: you can attack up to two close-together targets, AND, every target you fire at has a -2 on their Reaction roll (it's harder to dodge a scattered handful of little bullets, than a single one). That Remington 990 would have a damage code of "DV 7P(f), AP +6 vs Impact".
Finally, when firing a "Wide Spread", the gun is lettign those individual projectils spread out a whole lot. You can attack up to three targets with one trigger-pull, and each of them suffers -4 dice from their Reaction roll to not get hit. However, the damage code sifes penalties: -2 damage, +9 armorpenetration. Thus, the Remington 990 would have "DV 5P(f), AP +8 vs Impact".
...
If you want to fire a Remington without flechette rounds? Don't load flechette rounds. It'll deal "DV 7P, AP -1 vs. Ballistic". Weapons are only using Flechette rules by default, if you see that important little (f) in their Damage Value.
Do note, if you fire just a solid slug (the 7P, AM-1 version) ...? You can't set the spread at all. Like any other one-bullet-at-a-time gun, it's also a one-target-at-a-time gun.
Another note: the rules for Flechettes were changed. They're no longer just a +3 to the AP value, they're a full +5. So, again, a Remington 990 firing Flechettes in a Narrow Spread deals 9P(f) AP+4. The table listing "AP +2" is simply incorrect. Yayy, Errata. *sigh*