As title says. Thank you very much 
You need a “crit machine” as aether named it in his CA thread somewhere on the old forums (cant find it). Basically its a low level sniper with vladof barrel, preferably vladof sniper. Why low level? Because you can find a target and crit it without killing it for as long as you have ammo.
Once you deplete your ammo, you need to die / respawn so that some ammo gets added to your ammo pool. Repeat the process until satisfied with CA stacks.
Here is an example from aether:
Go to 9:30 to see how the stacking with crit machine actually works.
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Man of low moral fiber has a lot of videos showing how to do it in different spots
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Whenever I am playing sniper Zero, I usually use a low level Vladof Droog for the stacking. That is the non-unique purple Vladof sniper with the Vladof barrel. For prefix, go with fire rate/stability. I have a hard time managing the recoil, so I usually go with the stability prefix.
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