The New York Times reports that, as Adam Lanza killed 26 people (including 20 elementary school children), this is one of the guns he used, described as an M4 .223-caliber:
This is an M4.
Is this a good hunting weapon? Useful for personal protection, does it fit in a back pocket? What about for a neighborhood watch in suburban Florida?
I will never, ever understand the rationale for this weapon being available to the public.
12:15 PM PT: Some reports are that this firearm was left in the shooter's vehicle at the time of the attack; I'm not yet clear on the full story but have no reason to doubt this. I don't think that has effect on my underlying point; this weapon should not be available to the public.
12:34 PM PT: Title edited based on discussion in comments that this gun may not have been used in the shooting. I'm still not clear on this point but, regardless of that, all reports are that this firearm was seized by police in the aftermath. I'm not going to speculate on an investigation that I'm not part of.
1:00 PM PT: I'm not presently able to keep up with all of the news - I need to get lunch and then will try to find a reliable, comprehensive update on the ME's press conference under discussion in the comments; I haven't yet looked for a source on that but comments indicate that ME reported long gun usage in the shootings.