Primed Expel Grineer and Magnum Force are flex slots.
Flex slot options:
PHeat - Highest total damage, for unarmored enemies (doesn't increase Slash damage).
Faction(Expel) - Highest Slash damage, but only works vs one faction.
Magnum Force - Low but consistent damage increase. The -Accuracy can even be a positive, giving a larger AoE.
Galvanized Crosshairs - High damage bonus, but conditional, requiring specific actions to activate. I'm personally not a fan of aiming down sights with [Catchmoon].
Riven stat priority: CC = CD > MS = FR > Dmg
Riven replaces mod with same stats, in reverse priority. I.e: if you have a CC CD Dmg riven you replace a Dmg mod, [Magnum Force].
Sadly not polarity compatible with the standard Catchmoon build.
Detailed spreadsheets on part choices can be found in my Modular Weapons Guide: https://docs.google.com/document/d/1-74HS0rFLwRxDG4wz4PBRYEnQP2mzgis9HcD7nYGhKs