Here is a quick vid I made showing how fast you have to click after the damage or miss text disappears with the masters metal gun. I already posted this in another thread but I should have done it here first.
Lion Timed Crit vid:
http://media.putfile.com/Lion-Timed-Crit
Gun Used: Masters Metal Gun
One thing I want to mention is that it doesn't matter when the shot takes place as long as you click the mob at the right time your next hit will be a critical. So, you can run away to pot, hit something that is out of range, shoot another mob with the critical etc.