The guns weren't "fully automatic" just modified - something that is easy and legal to do.
No one has to carry all 23 guns and ammo to their room at once. Never seen anyone wheel a golf cart or long bag to a hotel room before?
And if you did some research that rapid fire of an assault rifle causes it to heat rapidly - there's video you can easily find of them glowing red-hot and actually catching fire. So if you want to actually handle them and not hurt yourself accidentally you fire a magazine, drop it, pick up the next one.
And how did he buy them all? The US government makes it remarkably easy and legal plus the guy had millions of dollars to spend. Just because his brother didn't know about his issues and gun fetish doesn't mean shit.
Remember correlation is not causation.