I know that they were working on high powered lasers some years ago (during Reagan's presidency) to use against satellites or ICBMs in outer space, but I don't know how far they may have developed them.
Lasers can be used over large distances in space, but are degraded by atmosphere. However, I would imagine it might be possible to use them over smaller distances of a few miles.
Again, I don't know for sure, but I would speculate that a laser needs to be fired up, and that would take a certain amount of time. Basically, a laser works by putting energy into a compartment where a coherent light beam bounces back and forth between the two ends of a cylinder until it gains enough energy to be released.
For a quick reaction, it might be necessary to keep the damned thing fired up all the time, just below the required energy level, but again this is pure speculation. If that were the case, probably you'd need some extra lasers, so some would always be ready to fire while others were brought down for maintenance. I THINK it would be possible, but I have no first hand information on it. This is all sheer speculation.
A gun that accelerates a bullet to a speed of 13,000 miles per hour in 0.2 seconds? Seems like a science fiction dream, but don't even blink -- with advances in rail gun technology, a new era of high-speed military weaponry is coming right at us, faster than a speeding bullet.