I do like the idea of a very short-lived shield, but perhaps basing it off of damage done is making it needlessly complex. Why not have a "SHIELD" bar, split in two. Hitting M3 activates the short-lived shield for a small amount of time (fraction of a second, perhaps?) and depletes one part of the shield meter. The meter recharges at a certain rate that I can't be bothered to think of right now. Maybe even make it a passive ability? Hitting M3 with any weapon active will activate the shield. Naturally you won't be able to spam it, but well-timed wrangles can potentially keep a sentry going under circumstances that would have normally killed it.