Vanguard help

I suppose it depends on how much weight you can carry before it screws over the ability reload. I think the sniper rifles tend to weight more than assault rifles?
I just went with sniper rifle as the additional weapon, for those fights and the ones with the kett "floaty ball shield" enemies... One with decent dmg and capacity.

You could always put the "tracking plasma" mod on your shotgun (or talon pistol).

This mod will drastically increase the range of spread-fire weapons against stationary targets.

I'd also recommend putting bullet penetration on such weapons so you can still hit enemies that duck into cover when you shoot at them.

The only weakness of such a weapon is against small, mobile enemies that avoid your bullets with strafing motions.

Eventually you can spec into more carry weight and also carry the higher quality versions of guns which are lighter. Then you could carry a secondary, longer range weapon.