Well it was a country music concert in the US, they probably were armed and no it would not. There is nothing that could have been done to prevent that tragedy, He had no criminal record or bad mental evaluations. He used firearms that are perfectly legal throughout most of the world (semi-auto rifles) and lets face it "bump" stocks are not high tech stuff and could easily be made on there own if someone wanted to they are just sliding stocks. He had an elevated position and was not visible to the crowd. No gun or gun control could have stopped that. That was a perfect storm of fuckery, and lets face it if someone wants to cause mass amounts of fuckery and plans for it no matter where in the world and what laws are in place they could pull it off. Throwing a fuck ton of Molotov cocktails into a crowded area with few exits from an elevated window probably would have the same effect. Renting vans, making bombs, etc.... The truly fucked will find away to bring about their fuckery.