There were the two Glocks, of course, which almost go without saying. Nine millimeter has good stopping power, but they’re also well-weighted, industry standard sidearms, with great predictable handling and reliability.
The Remington 870 Tactical? It’s legendary for its reliability and robustness, the single most successful pump action shotgun in modern history. It may not put as many rounds through as quickly as a semi-auto or full-auto combat shotgun, but it’s stone-cold reliable. There’s a reason it’s the weapon of choice for law enforcement.
The AR-15 is the only debatable choice...for close quarters a semi-auto bullpup like the FN2000 might be arguably better, but a bullpup could lose balance and would certainly become unwieldy if you intended to use a large-capacity 100 round drum magazine. Sure, you can change out magazines in less than two seconds, but in the absence of cover, that would be a very long two seconds. Of course, ever since the size limitations on semi-auto magazines were allowed to expire, this is a moot point. Given the intended purpose, the combination was a logical choice, although the tendency for the AR-15 to jam when used with large capacity magazines did end up being a factor.
He’d done his research, I’m sure, assessing the reviews that can be found everywhere online and in enthusiast magazines. His weapon selections show thoughtfulness and informed consideration.
He also knew that Colorado is a state where access to firearms is easy. In fact, Colorado has laws on the books forbidding gun registration in any form by any locality. It’s really easy to buy a gun there. Further, laws establishing concealed carry are on the books. There could easily have been a gun-owner with a concealed weapon on the audience. In anticipation of this, he did the smart thing.
He purchased tear gas, which in a closed space would quickly render all but those wearing gas masks unable to clearly see or focus. He armored himself, wearing a bulletproof vest with groin and neck protection. His assumption, a logical one, was that any armed member of the audience would be unlikely to successfully acquire a target in darkness while incapacitated by tear gas and in the chaos of a panicked crowd. Between the tear gas, the optimal weapons selection, and the body armor, there is really no rationally defensible scenario in which he could have been stopped. Barring an audience member with a gas mask and a long gun, there was little possibility of preventing him...legally...from carrying out his attack.
In fact, given the location and Colorado laws about open carry...meaning you can carry a weapon openly and in public...he was completely within his rights right up until the instant he opened fire. Had someone tackled him before he first discharged his weapon, it could have been considered an assault.
Unlike our culture’s non-response to these regularly recurring events, it was all very well thought through. And both our society and its law were...right up until that moment...very much on his side.