I'm pretty sure just keeping it on is overall worse.

auto attack nearly hits 1 for one on skills and is 100 potency, we lose it.
testing it out the barrel seems to do nothing to turrets so can ignore them.

to gain 30% on potencies of 140,180 and 200

potenices with auto attack would be 240,280,300

potencies with no auto attack and 30% barrel 182,234,260

for being able to use a 180 potency move every 20 seconds you sacrifice about 8 shots of 40-60 potency.

It would be a good increase in damage if we could put it on then take it off after a combo of moves but now you cant take it off untill the recast is finished.