This build is conditional in that you have to be aiming down sights to trigger [Critical Focus], this build is optimal against Grineer targets due to the Heat and Viral damage, Heat reducing their armour and Viral increasing health damage done.
We get Viral damage by using both Cold and Toxin status mods: Polar Magazine and Venomous Clip.
Primed Rubedo-Lined Barrel can be replaced with the regular version and Critical Focus replaced with a 60/60 archgun cold/toxin status mod for less DPS although is more affordable if trading for the mods.
[Dual Rounds] is to provide multishot to fire more projectiles more often, with [Parallax Scope], [Hollowed Bullets] and [Critical Focus] contributing to our critical chance and critical damage.
Other Factions:
For Corpus we will want Magnetic to weaken their shields, simply replace Venemous Clip with [Electrified Barrel].
For Infested, we will want Gas as it is most effective against most types of infested health, simply replace [Polar Magazine] with [Combustion Rounds].