Droog or Strike
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Taken from the master himself:
crit machine... get 2.
for the vladof one get a droog (vladof barrel) with the bolshy accessory (mag size) +vladof stock;
for the dhal crit machine get a strike (hyperion barrel) with the liquid accessory (stability) a dhal grip and hyperion stock. (do not get a dhal stock on this gun because it adds a round to the burst, which in this case you don't want)
if you can only get one get the droog. the dhal crit machine is the mob runner, the vladof works too but the dhal one upkeeps your stack in a way that prevents you having to make a stack dummy in the field. the droog works in both situations but the strike doesnt boss stack effectively. the strike is a much better mob crit machine than the droog tho because you don't need to dump 50 bullets at a time into ca once your running a zone.
A bit more of information on crit machines as stated by Dotanuki:
Ok, on critmachines
The Strike, Liquid prefix/Stability accessory (the foregrip). Suppressive does not make much sense since fire rate does not affect burst fire.
The Droog, Bolshy prefix/Mag size accessory and/or Skorry prefix/Fire rate accessory. With a 91% sniper ammo relic, Bolshy will drain full ammo faster because it gets one less reload over Skorry. But for the ammo you get when you kill yourself you only get one reload either way, so Skorry is faster. So when boss stacking, ideally, Bolshy for the entire pool, and then swap to Skorry when you kill yourself for ammo. But for stacking in the field, Skorry is better/faster.
As I mentioned above, the Invader can also work as a CA stacking sniper, as well as CA main killer weapon.
What you basically want in a Critmachine is a low level (normally level 8 - minimum level for snipers to drop from loot chests) sniper with fast fire-rate and low recoil as possible so you can fire into the critical hit spots of enemies as accurate and as fast as possible so you can stack CA and then switch to a more powerfull sniper and start killing enemies. With this in mind, both the Droog and the Strike fall into this category and are the preferred snipers for this job.
Best parts: All Vladof for the Droog; Dahl grip/Hyperion stock for the Strike
Best prefix: Skorry or Bolshy for Droog; Liquid for Strike
Drops: Best place to farm them is on the Southern Shelf map, Normal mode
EDIT: I have found some level 7 snipers in the Sanctuary or Three Horns Divide vending machines when a new character arrives for the first time, but their parts were not as good as the level 8 Kot Droog that I use. I’m still looking for a better accessory.