There is nothing in the article that says the bullet trajactory was vertical. Yes, "roof" is "up" from a child sitting in a pew, but NOT straight up. That means absolutly nothing when we are discussing ballistics. A Sierra Match King rises several feet over the ground before coming back down and thru the target.
Ever heard of "Plunging Fires" and "Beaten Zones"? Don't you think you should tell the US Army about your vastly superior knowledge? They have been teaching machine gunners about this since 1900. Pity you weren't there to tell them they are wrong and that gunfire coming over a hill is entirely harmless...
The discussion was about bullets fired straight up (or close) coming back to earth with enough force to penetrate a roof and kill someone inside. It referenced Mythbusters who tested just that hypothesis. Clearly, the bullet was probably fired at the roof and was a deflection.
While I probably can’t teach the army much about ballistics, I can calculate muzzle velocities and joules of energy for different rounds.
Absurd ideas like the bullet increasing it’s velocity because of gravity need to be challenged.