The Horus Heresy is a narrative game. Nothing kills a narrative faster than arguing about whether a Meltagun is Assault 1 or Heavy 1 for three hours.
For those looking for rules and settings for the Horus Heresy, Games Workshop has released several publications and supplements. Here are a few: horus heresy 20 rules pdf better
Many hardcore players prefer paper at the table (no iPads). A "better" PDF often comes in two color variants: The Horus Heresy is a narrative game