If Legolas with a rifle is something like Hawkeye or Deadshot and he had a regular mid calibre rifle, he could probably carry more magazines on him with more rounds than he could carry arrows. The reloading might be an issue, but the higher fire rate than a bow should make up for it. A proper sniper would be excessive for killing via headshots, and ammo would be a much more limiting factor. Just handing him a rifle probably wouldn't accomplish much though, since he's trained with a bow.
Orcs typically cluster though, so the LMG with an extra drum or two would probably make a bigger impact. But a couple grenades would have a bigger impact still.
Orcs typically cluster though, so the LMG with an extra drum or two would probably make a bigger impact. But a couple grenades would have a bigger impact still.