Well he's not the only mass shooter who's chosen that rifle. It's the weapon of choice. And, I've not seen any evidence that a handgun would be more efficient in killing the maximum number of people. I have a friend who served in Afghanistan. He said there is a reason all our guys in Afghanistan and Iraq clear buildings with their rifles, not their sidearms. Because sidearms are a LOUSY substitue for rifles.
I was a US Marine. We use rifles over handguns because they are A. More accurate in trained hands and have a much larger effective range and B. More effective against targets that may potentially have body armor. Neither of those are a factor in the shootings that have gone on.
For what he was doing, point blank unarmed civilians, a shotgun would have been much more devastating, and the pistols are much more easily concealable, can carry 2/3 as much ammunition in a single magazine, and are just as deadly (at that range)
FYI, teams that are primarily engaged in CQB (close quarter combat, I.E. small room clearing) use extremely short M4 Carbine variants (the SEALs variant is called the Mk18 CQBR). It's illegal for a civilian to own a short barreled rifle without jumping through hoops.
And I don't know about your friend, but most Infantryman aren't issued sidearms.
My question is, why in one post, you say that you don't know shit about firearms, but then proceed to attempt to lecture like you know what the fuck you're talking about?