The Surefire Backup is a great light! It has a high output mode (200 lumens) that is blinding and a low power mode that is great for general use (5 lumens). It only uses one lithium CR123 battery which keeps the size down. Run time for the high output is 1.3 hours and run time for low output is 40 hours (according to Surefire's website). Overall length is 4.4 inches which is right around the width of the average hand. I have the older model and I carry it in my back pocket without any issues. The only issue is these lights get HOT after a while. I'm sure that is the case with any of these high output lights though.
The ATF should be a convenience store instead of a government agency!