I like the beam idea, and it matches the common conceptualisation of a railgun style weapon. You aren't actually seeing the slug, but the path of ablative atomic particle damage it leaves behind as a wake. I don't think "lasers" are actually light based, as some ships can outrun them.
Probably more like a laser generated field of plasma that is produced in a chamber and then shot out of a barrel like a regular round, just made of really hot stuff. The actual beam weapons themselves---those could be laser light, and they along with a railgun would appear to fire and hit in the same moment, since the scale of the battle theatre is too small to observe the movement of near or at-light speed.
