Sometimes it takes longer than (what I believe to be) the expected time of ~1s, and sometimes it just doesn't seem to activate at all. Please just remove it.
Let's get away from these gotchas and just make things a bit simpler. The game's already plenty hard enough without this kind of stuff getting in the way.
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Remove specific weapons for specific roles altogether. Make barrier any weapon 3s aim down sights to activate, overload hip-fire shots and unstop a skill. Done, Or primary slot anti barrier, secondary slot overload and unstop heavy. So many ways to do it without this narrow focus, even as simple as champion stun occurs after a certain amount of damage🤷🏻♂️