problems not talked about, not as many pellets, smaller diameter strike area. 20 gauge ammo after shft might be near impossible to find. Using 1 buckshot increases the pellets by 2 with negligible diameter in pellets but a slightly faster fps.
Mag fed shot guns and felt recoil can be fully compensated for in full auto aa12 look at video guy dual shoots them. Truely a fearsome close range defense system, they even have a remote turret mount dual aa12 for apc with control to fire one or the other so non lethal loads can be used until such time as force needs to be escalated.