Mod types are roughly color-coded. Red is Damage, pink is Multishot, orange is Crit, and green is Elemental or IPS. Variant mods are counted the same (Amalgam Serration → Serration), and lower-tier version are rounded up (Heated Charge → Primed Heated Charge).
Rifle Builds
Only 27 different mods are present in the top 80 Rifle builds on overframe.gg.
Split Chamber is present on every single build. Serration is present on all but one, a build for Eidolon hunting with Chroma.
9/10 builds are Crit-focused, and Heavy Caliber and Vigilante Armaments are present on 2/3rds of all builds. 1/3rd of builds use Hunter Munitions. 1/8th use Hammer Shot.
Every single build uses at least one 90% or 60/60 Elemental mod, 2.25 of them on average.
Damage, Multishot, and Crit > Elemental > Other
Secondary Builds
Only 25 different mods are present in the top 99 Secondary builds on overframe.gg.
Hornet Strike and Barrel Diffus...