I imagine that the bullet wouldn't just burst into powder, seeing as what it's impacting is a blade consisting purely of self-contained plasma. The Lightsaber would probably turn the bullet into molten slag, if not disintegrating entirely due to the immense heat. I wouldn't even say equipment is the only factor that matters here, such as the abilities and reaction time of each respective party. Of course though, the amount of bullets the Spartan (what kind are we talking?) can send flying would indeed be an important factor.