I am looking for example of some specific guidelines like, how many days do I need to wait before I can point out how freaking explosives and automatic weapons shouldn't be as easily obtainable as they obviously were in this specific case.
On what do you base the claim that they were "easy" to obtain? Did he even have explosives? It's possible to make explosives out of common items (like urine, wood, and sulfur). And we don't know how hard it was for him to obtain an automatic weapon. Did he buy it? Did he make it? Did he steal it? All we know now is that he had one. Perhaps he only obtained it with great difficulty.
So here's my proposal. Let's wait until after the investigation has finished and we know what happened. In particular, we should know how he acquired a fully automatic weapon. Once we know and aren't just speculating, then we can talk intelligently about how to make that more difficult in the future.
A side effect of waiting for the investigation to finish is that they will release the bodies of the victims to the families so that the funerals can occur. And the surviving victims have time to leave the hospital and meet with psychiatric professionals who specialize in handling trauma.