Don’t waste your ammo on hordes because you will run out of ammo constantly. You need to learn to melee. Your gun should only be used on elite units and ranged units.
Depends on the gun, some weapons are very ammo efficient in both firing and with ammo pickups. If you have people on your team who are good at and have good horde clear weapons, yeah, no reason to bother, but that won’t always be the case.
I have another one.
If you are the only veteran in your squad, pick a good lasgun and the Counterfire talent. Use you active ability every time you enter new unexplored area, and pick off all ranged units. If you mix the easy to kill trash ranged units, you can clear all of them or majority during one ability use, even if you need a reload. Shotgunners are the priority to kill, other elite gunners next in the priority queue.
This way you will make a far bigger difference to your squad than with a plasma or bolter. if you have another veteran in your squad, THEN you can think about having a different specialist in your team.