Honestly (and I'm not trying to bash you in any way), I think making tracer missile an instant cast is a terrible idea. Here is my reason why: every class in the game can have important casted abilities interrupted (minus ravage, but if you're smart you'll just stun or kb a ravage). Interrupting TM is a reward to smart players. I cannot tell you how many times I've been casting TM, and my target didn't even bother to interrupt it; they got what they deserved, I'll leave it at that. If someone interrupts TM we are, for the most part, shut down.
Course, if Grav Round/Tracer Missile was instant they would just move on to interrupting Full Auto/Unload.
Interrupting a single ability should not immediately shut down a class. However, that is what interrupting Grav Round/Tracer Missile does since so many of our abilities are tied to it.
Make GR/TM instant and reduce its damage and either 1: Up the damage of our other abilities to make up for it, or 2: Return the proc possibility of CoF/Barrage back to 3 seconds instead of 6.