To be fair, he has a small hiccup in the loading portion of things— he's never used a rifle like this before, and the charge wasn't what he was expecting when he asked for ammo. Adrenaline is keeping his hands steady, but he's very aware of every second that thing is still stomping around. "What kind of futuristic bullshit is this? Jeez...."

When he has it loaded, he scrambles for better cover (height advantage: this is not that hard to do), and squeezes off a volley of shots. He might not be a sharpshooter, but the weight of a firearm is familiar, and he doesn't hesitate. It's not especially pleasant, but, well, not like he's got much of a choice.

He knows he doesn't have much in way of ammunition, so he's sacrificing speed in favor of a) not missing, and b) giving Cayde some breathing room.