As a Gunslinger, a Juggernaut and an Assassin, i've found that pretty much the outcome to a tracer missile spam fight is who gets the the jump.
My jug and assassin got enough interupts to mess up a merc in no time if im ontop of them before they start spamming, while my Gunslinger can outright kill them if given the 3 seconds to set up burst. However if im caught at range, or they attack me first and get their their button mashing going i'm dead 90% of the time.
For my Gunslinger, Take first hit ... 1.5s to enter cover, 1.5s to set up an attack of some sort ... theres 3 hits and a 4th incoming before i've even hit them once and then im usually dead.