The grenadiers in my squad (Mech Engineer) usually carried 2-4 Illumination & 3 or so smoke in the vehicles: As nobody wore the grenadier vest when I was in Iraq. But the bulk was HE or HEDP 6-12 rounds carried in the issue bandoliers ( My grenadier carried them in 2 aftermarket pouches). I preferred the grenadier on my fire team to carry the HE as the HEDP seemed not to be as good round for frag purposes.
As for the more exotic buckshot cartridge, the only guys I saw with it were SF since the 80s. I've fired a few over the years and they were ok. The pellets were about the size of #4 buck and it only had 20 or so pellets. Never seen a flechette round I believe they were only issued ion a experimental basis in Viet-Nam.
The "Text book" load out is 18 rounds, only 6 of which are HEDP. The rest are assorted flares and smoke. Thats what my unit provided, as it was carry over "UBL". For those that dont know what UBL is, its universal basic load. Basically, a Military units SHTF ammo that is ONLY to be used in Combat.
So, "on patrol" in Iraq, I would carry the 6 HEDP, and 4 white smokes.
No 12 gauge adapter, buckshot round, etc.... Remember that you have a 5.56 attachment..