re: CO shooting
Two main factors I suppose.
For about ten years I was carrying a S&W M296 Airweight Centennial in .44 Special. Love everything about it. Still carry it every now and then when absolute lowest weight is necessary. I'm actually a decent DAO shot, benefits of being 53 and shooting since you were a child, but the M296 has a fairly stiff pull and I find that particular one difficult to stage. Maybe it is even a function of the light weight of the gun? Anyway, at the range and in good light and nobody shooting at me, I can get solid chest hits with that gun out to at least 25 yards. While I can shoot my old S&W M640 Centennial out much further, the M296 really only seemed happy fairly close. Which never seemed like much of a liability since a civilian is only going to engage up close... most of the time.
When the shooting happened, I "put myself in a seat there." If I was right up front, I would probably be dead before I could really engage. Unless I was just flat lucky. But if I was in the middle or further back, it seems like there is a real chance to get down on the floor, crawl to the far end of an aisle, and then engage from low level. But at what distance? The guy threw smoke cannisters out into the seats. So from 40 yards away, in the dark, through shifting smoke, under crazy amounts of pressure, with a yahoo on stage shooting and killing people every second? I came to the conclusion that I might get a hit or two but it would definitely be a crap shoot. So I wanted more usable range.
Which leads directly to lesson number two. I love .44 Special and .45 ACP. It's in the blood. But the larger and slower the round, the easier it is for any kind of vest to stop. I had already been spooked by the Tyler, TX shootings. Mark Wilson engaged that reject killing people with a rifle but the handgun rounds failed to penetrate the guy's vest. (Colt 1911 .45 ACP.) And Wilson died because of that. The CO shooter also had on tactical type clothing. Some type of vest. And helmet. So even if my .44 Special CorBon 200 grain solid copper JHP hit the bad guy right in the chest... he might well just keep on firing. Or turn towards me and kill me.
So my lessons that led to going to the HK P2000 LEM in .357 SIG... I wanted more range. I wanted more tactical penetration. I wanted to be able to get chest hits out to 50 yards or so and have those hits penetrate everything but the very top level vests. I'm not a huge fan of "firepower" but I also had to admit that 12+1 compared to 5 sounded better if I was really going to try to shoot somebody that was firing a rifle. I'm guessing the five rounds would go bye-bye very quickly. I really don't want to be reloading while everything around me is disintegrating under rifle fire.
Honestly, I like .357 SIG a lot. I've shot it for years in a variety of guns. I've reloaded thousands of rounds for it. I've shot various types of animals and always been impressed. Much more so than the same types of animals hit with a .45 ACP. And maybe it is because I'm an old revolver guy... but I really, really like the HK LEM system. I'm not anti-Glock, I own several. But I will take the LEM system over something like the Glock any day. Especially if I'm going to conceal carry it.
Hope that answers your question.
Gregg