Replace Electrified Barrel with Marked Target for same status chance as the top ranked build. Replace Rubedo-Lined Barrel with [Primed Rubedo-Lined Barrel] for basically the exact same thing as the top ranked build, but Viral + Rad rather than Corrosive + Heat + Rad. If you want the ladder three status effects, then use Electrified Barrel in place of Polar Magazine. To make those final 2-3 changes, you will require 1 forma and an extra 40000~ endo (most of it comes from maxing out Primed Rubedo-Lined Barrel.) Alternatively, if you're fine with the status chance, then only replace [Rubedo-Lined Barrel] for maximum raw damage, and throw in another forma and replace [Magma Chamber] and Contamination Casing with Combustion Rounds and Venomous Clip.

Finally, if you want even more status than what the top-ranked build has, replace Electrified Barrel with Marked Target, Polar Magazine with Hypothermic Shell, requiring no extra forma (assuming you don't replace Rubedo-Lined Barrel with [Primed Rubedo-Lined Barrel]) but providing an extra 30 status chance in place of a good amount of damage. If that's worthwhile to you, go for it. Ff it's not, then just stick with replacing Electrified Barrel/Polar Magazine with Marked Target (or don't replace at all) and add 1 forma so you can replace Rubedo-Lined Barrel with [Primed Rubedo-Lined Barrel].

Happy building :)

Notes: [Dual Rounds] and [Automatic Trigger] can be farmed from Caelus, Uranus, Rotation A. Contamination Casing + Charged Bullets + Magma Chamber are exclusive to Fomorian/Razorback, but are only 10 plat each; just grind out an extra Dual Rounds, it's worth like 40 plat, or use w/e plat-farming method you prefer to get the necessary 30 plat. Electrified Barrel is from Caelus, Uranus, Rotation B. Polar Magazine is farmed pretty easily from Salacia Neptune/Erpo, Earth. [Rubedo-Lined Barrel] can be gained from Caelus, Uranus, Rotation A, but also from Hellion Dargyns, who are similarly on Caelus, Uranus, Rotation A.

In total, this should take maybe 5 hours of grinding even for unlucky people + 30 plat. Very cheap forma-wise (0), very cheap endo wise (4k), very cheap plat wise (30), only a little bit of a grind. Also, if Fomorian or Razorback happens to be active, you can grind those out to get [Contamination Casing] + [Charged Bullets] (Fomorian) and [Magma Chamber] (Razorback) to reduce the plat cost.