Maybe, if you have your launcher sticking outside the shield or you're halfway through it yourself but if you fire a rocket straight at a shield...it's going to explode on the shield.
Not quite. Sometimes shooting out of any of the spawn shields or the warpgate bubble with explosive projectiles is buggy and one gets blocked randomly. But aside from that, almost every single spawn shield in the game can be shot out of with anything. The newer spawn shields at the top of air towers can't be shot out of at all, of course.