You can check this in buy/sell or equip menu - just select riot shotgun and scroll ammunition types. I`m no military expert, but as it is flechettes have lower range (15), scatter more and are much more damaging (per shot - 1 round contains 8 flechettes, while single slug is just a single slug). Therefore it would IMHO ore logical if flechettes required close skill, and slugs - assault. Or they could use both close skill.