Something I've noticed is that all of these hpa engines, whether it be polarstar, inferno, mk2, etc, don't use a beveled nozzle like most aeg manufactures do. I was seeing problems with this on my inferno, where the nozzle would have about a .5mm play to either side while meshing with the hopup unit. I'm not sure if this is the case with a prowin, but it was causing a lot of consistency problems if the o ring wasn't properly sealing.
Other things I can think of is inconsistencies in hopup pressure, it could just mean that you need to tighten the hopup arm adjustment knob screw.
Another thing I can think of is your regulator not providing consistent fps. Depending on your regulator it may just not be broken in, or it was over lubed, which is quite normal if you purchased from amped.
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