Light (laser beam) travels at some 186,000 miles per second. That translates to a really, really fast particle or mesh that stretches from its origin point (barrel of gun) to its destination (impact point). The engine, while quite powerful, may miss a thing or two calculating what is collided with when moving that fast.
That situation may (read: WILL) require a line trace. Spawn your laser particle system along the trace line to simulate a laser.
A missile generally travels at around 200 miles per hour, quite a bit slower than a laser beam. The engine can and will calculate any and all collisions this missile has within the collision channels it is checking.
This situation may use a mesh/particle/what-have-you.