There were what appeared to be two row of three bullet holes each in the window of the Banner hospital in Ft. Collins that was shot on 9/12/14. That suggests that the shooter was shooting short bursts from a fully automatic assault rifle like an M-16 that allows you to keep firing quickly without having to pull the trigger again and again. Individuals aren't generally allowed to own fully automatic assault rifles, but can own semi-automatic versions of them like the AR-15 that can only fire as quickly as you can pull your trigger finger, as Foxfire had mentioned earlier. Some semi-automatic assault rifles can be 'modded' back to fully automatic by someone who knows what they are doing.
All we know about the bullets fired at the hospital is that they were too light or slow to go thru the windows. 22 cal.? [ETA: Or a frangible round of a higher caliber? As I'd mentioned in an earlier post, I've heard that some kinds of frangibles won't penetrate windshields.]
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Photo credit 7 News ABC, KMGH