This is Willow's gun - "Little Jimmy". He's a custom built light machine gun that she modified herself. (She has another gun - a Defiance Short shotgun that she wears like sidearm, but she doesn't fuss over it nearly as much.) She gave it gas-venting, a weighted barrel, a custom grip, and just to top it off, made it proportional to herself so it wouldn't look like she was lugging around a little Nerf gun. (Though, she's since added a bayonet as well.)
I was looking at the stats for a Troll-sized weapon, and trying to figure out exactly how big Little Jimmy would be in comparison to an average pistol. You can see the pistol just below him for a good comparison of size. All I can say it - yikes.